The Philip Morris Company did not like to talk about what went on inside its lab in Cologne, Germany, where researchers secretly conducted experiments exploring the effects of cigarette smoking.

So when the Justice Department tried to get its hands on that research in 1996 to prove that tobacco industry executives had lied about the dangers of smoking, the company moved to fend off the effort with the help of a highly regarded young lawyer named Kirsten Rutnik.

Ms. Rutnik, who now goes by her married name, Gillibrand, threw herself into the work. She traveled to Germany at least twice, interviewing the lab’s top scientists, whose research showed a connection between smoking and cancer but was kept far from public view.

She helped contend with prosecution demands for evidence and monitored testimony of witnesses before a grand jury, following up with strategy memos to Philip Morris’s general counsel.

The industry beat back the federal perjury investigation, a significant legal victory at the time, but not one that Ms. Gillibrand is eager to discuss.

Now in the Senate seat formerly held by Hillary Rodham Clinton, Ms. Gillibrand plays down her work as a lawyer representing Philip Morris, saying she was a junior associate with little control over the cases she was handed and limited involvement in defending the tobacco maker.

But a review of thousands of documents and interviews with dozens of lawyers and industry experts indicate that Ms. Gillibrand was involved in some of the most sensitive matters related to the defense of the tobacco giant as it confronted pivotal legal battles beginning in the mid-1990s.

Ms. Gillibrand, who worked at the Manhattan firm of Davis Polk & Wardwell from 1991 to 2000, eventually oversaw a team of associate lawyers working on Philip Morris cases, according to a colleague, and was a frequent point of contact between the firm and Philip Morris executives.

In addition, Ms. Gillibrand represented Davis Polk on a high-level Philip Morris committee whose work included shielding certain documents from disclosure, according to several lawyers and industry observers. Serving on the panel placed her alongside some of the country’s top tobacco industry lawyers.

He stressed that like other tobacco lawyers, she was not at liberty to discuss her work for Philip Morris because of attorney-client privilege.

But those who recall Ms. Gillibrand’s days as a young lawyer say she was capable and eager as she plunged into the high-stakes and lucrative world of tobacco defense work.

“The client was always in her office,” said her former Davis Polk colleague Vincent Chang, who spoke glowingly of Ms. Gillibrand. “She was probably accorded more responsibility than the average associate by far.”

Of course, many lawyers, including some who now serve in the Senate, have defended unpopular clients. Still, in an approach that was not uncommon at law firms that represented tobacco companies, lawyers at Davis Polk were permitted to decline work on the tobacco cases if they had a moral or ethical objection to the work, Mr. Chang said.

Asked whether Ms. Gillibrand had any misgivings about representing the tobacco company, Mr. Canter responded by e-mail: “Senator Gillibrand worked for the clients that were assigned to her.”

Ms. Gillibrand was never the lead lawyer on the tobacco cases, which at Davis Polk drew on the work of dozens of lawyers and staffers. Robert B. Fiske Jr., a former Whitewater prosecutor and a Davis Polk partner, was the top lawyer among the approximately 20 at the firm working on the Philip Morris defense on the perjury case. Ms. Gillibrand’s hourly rate — $305 in 1995 — put her in the middle range of reimbursement for associates on the case, according to a tobacco industry document.

Mr. Fiske declined, through the senator’s office, to be interviewed about her work for Philip Morris, but released a statement calling Ms. Gillibrand “smart, hard-working and thoughtful.”

During her most recent congressional race, Ms. Gillibrand, who is a former smoker, accepted $18,200 in campaign donations from tobacco companies and their executives — putting her among the top dozen House Democrats for such contributions. Many Congressional Democrats do not accept tobacco money.

Mr. Canter said the senator should be assessed based on her record in Congress, where she has voted against the industry’s interests on several occasions, including supporting cigarette tax increases to help expand children’s health care.

And Todd Henderson, an assistant professor at the University of Chicago Law School, argued that it would be unfair to assess lawyers by whom they represent. “Nobody would want to live in a world in which lawyers are judged by the clients they take,” he said.

Limiting Evidence

A scion of a prominent Albany political clan, Ms Gillibrand graduated from law school at the University of California, Los Angeles, in 1991 and took a job at Davis Polk, a firm that had worked closely with the tobacco industry for decades.

Ms. Gillibrand was working at the firm during critical years for the tobacco industry, as the public tide was turning against smoking, and leading Democrats in the Clinton administration and Congress pushed for a more aggressive stance toward cigarette companies. At the same time, plaintiffs’ lawyers were beginning to chip away at the industry’s time-tested legal strategies.

In 1994, executives of the nation’s largest tobacco companies, including Philip Morris, prompted anger and disbelief when they swore before Congress that they did not believe smoking was addictive or that there was a proven link between smoking and cancer.

That appearance intensified criticism of the industry and scrutiny by federal prosecutors and ultimately led to a broad criminal investigation by the Justice Department into whether the executives had perjured themselves.

The government sought reams of internal company records to determine whether the tobacco executives had lied. There is no indication that Ms. Gillibrand ever discussed the case with William Campbell, then the Philip Morris president and chief executive, who was among the subjects of the perjury inquiry. But Philip Morris internal records show that the company’s top lawyers entrusted her with several essential elements of the case.

As a member of the Eastern District of New York Subpoena Working Group, Ms. Gillibrand helped limit what evidence the government obtained. She also monitored the testimony of witnesses who appeared before the grand jury and wrote strategy memos to the Philip Morris general counsel, Ken Handal, analyzing the witnesses’ statements and their impact on the investigation.

Her travels to Germany took her to the Institut Fur Biologische Forschung, or Institute for Biological Research, a laboratory that Philip Morris had set up in Cologne, which has been criticized by antitobacco activists and cancer doctors. The establishment of the lab overseas, where topics of study included the role of tobacco in cancerous tumors, had allowed the company to keep conducting research there, beyond the reach of the United States government, news media and plaintiffs’ lawyers.

Ms. Gillibrand learned so much about the laboratory’s inner workings during the criminal investigation that by 1997, records show, she provided Philip Morris lawyers with a list of questions about the German lab to help them prepare company witnesses being called to testify in civil cases in Minnesota and elsewhere across the country.

At the laboratory, she interviewed Dr. Max Reininghaus, the general manager who oversaw the experiments, and reviewed lab personnel records that had been sought by federal investigators.

In 1998, when the case reached a turning point as one tobacco company, the Liggett Group, considered cooperating with prosecutors, Ms. Gillibrand was one of a handful of lawyers for Philip Morris privy to the unsuccessful efforts to dissuade Liggett from breaking ranks with the other cigarette makers.

She was also among the small group of Philip Morris lawyers involved in the effort to contain the damage the defection could do to other companies in the tobacco industry, pushing to prevent Philip Morris from disclosing any documents that would violate the confidentiality of the other co-defendants.

“She clearly was more than a lowly associate lawyer on the case,” said Anne Landman, a tobacco document researcher who has testified against the industry and edits Tobaccowiki.org, a Web site that provides analysis of tobacco documents. “Philip Morris showed deep trust in her and brought her in on sensitive legal matters that were of great importance to the company.”

In the face of the vigorous counteroffensive from the industry, the Justice Department abandoned its criminal inquiry in 1999 and decided to bring a racketeering case in civil court, claiming that the cigarette companies conspired for half a century to mislead the public about the dangers of smoking.

Ms. Gillibrand did not work on the racketeering case, on which other law firms took the lead. But when Judge Gladys Kessler of Federal District Court handed down her landmark decision in that case in 2006, finding that the tobacco companies had conspired to defraud the public, she based the ruling in part on the business practices Ms. Gillibrand had delved into during the perjury case. The judge cited Philip Morris’s use of the German lab as a way for the company to suppress evidence and scolded the company for concealing information from consumers and government regulators.

Asked last week whether Ms. Gillibrand agreed with the judge’s decision, her spokesman replied: “Senator Gillibrand did not work on that case and is not familiar with its details.”

A Rising Star

Ms. Gillibrand was also deeply involved as Philip Morris and other cigarette makers confronted another challenge: mounting accusations that the industry was abusing the attorney-client privilege to prevent disclosure of damaging research and other sensitive documents.

Legal experts and a Congressional committee said that for decades, the companies had misused the attorney-client privilege to try to conceal scientific information that was damaging to the industry. The lawyers, for example, participated in overseeing scientific research projects that they could then keep confidential. But in the 1990s, government and plaintiffs’ lawyers began directly challenging this protection.

The state of Minnesota, as part of a lawsuit seeking to force tobacco firms to pick up the state’s cost of treating smoking-related illnesses, objected to the companies’ claim of attorney-client privilege, invoking what is known as the crime-fraud exception: essentially, an assertion that the privilege did not apply because the lawyers were being used to help the companies commit fraud. A Minnesota judge agreed, saying that Philip Morris had engaged in an “egregious attempt to hide information” and, in a major blow to the industry, eventually forced the release of some 30 million pages of documents from industry files.

Philip Morris and the other companies subsequently settled the Minnesota case for $6 billion in 1998.

But with the industry facing other lawsuits around the country, Philip Morris turned to a committee it established to handle issues surrounding disclosure of other documents. In some instances, the committee sought to determine if certain documents had been improperly shielded under attorney-client privilege rule. But the committee also worked to protect other industry documents from being released, a practice that drew harsh criticism from lawyers and others who took on the industry.

Clifford Douglas, who served as a lawyer for the Congressional task force that looked into the tobacco industry’s practices, said, “The crime fraud committee was charged with preventing plaintiffs or the government from seeing sensitive documents that Philip Morris wanted to keep secret.”

Some of the nation’s most prominent tobacco lawyers from several prestigious law firms had seats on the committee, known as the Philip Morris Crime Fraud Issues Committee. And so did Ms. Gillibrand, who was already seen as a rising star among her colleagues at Davis Polk.

Mr. Chang, who worked with her at the firm, said it was telling that Ms. Gillibrand would be assigned to the panel along with “the linchpins of the tobacco defense bar in the entire country.”

“That’s certainly an indicator of the kind of respect that she was accorded at Davis Polk that they would choose her — a relatively junior associate — to be on a panel with some of the most prominent senior tobacco lawyers in the country,” he said.

Leslie Wharton, a senior counsel at the Washington law firm of Arnold Porter L.L.P. and a member of the crime fraud committee, said that although Ms. Gillibrand had less experience and stature than other lawyers on the panel, she was assertive, deeply involved and very effective in advocating on behalf of Philip Morris.

“She did more than pull her own weight,” Ms. Wharton said. “We handled highly specialized issues on a whole variety of cases, and she was a full partner in everything we did. She worked as hard as anyone and was a very capable, smart lawyer.”

Much of the committee’s work remains sealed, but internal documents indicate that the committee had wide latitude and “should be consulted with respect to just about any privilege issue that might arise in any case.”

A Philip Morris spokesman declined to discuss the committee or when it was formed.

Helping With Strategy

At Davis Polk, lawyers not only represented Philip Morris in litigation, they advised the company on business strategy, including how to protect the image of the cigarette company and how to deal with concerns about the effects of its products. This approach reflects, in part, the longstanding closeness between the firm and tobacco makers. But it also raised concerns among critics that the lawyers had crossed a line, and were essentially becoming agents in the business operation.

There were instances, for example, when Ms. Gillibrand was called upon to help the company deal with mounting public unease about its product and practices, according to interviews and a review of industry documents. Ms. Gillibrand was also schooled in some of the chemistry of cigarettes.

In 1998, for example, Roger G. Whidden, Philip Morris’s vice president for worldwide regulatory affairs, wrote Ms. Gillibrand a letter along with a draft document containing proposed responses to possible questions from reporters about nitrosamines, a cancer-causing agent in cigarettes.

In the letter, Mr. Whidden tells Ms. Gillibrand that the draft was prepared “on the basis of conversations” with her and others at Philip Morris, and asks her to review it. The suggested answers state that Philip Morris is working to reduce the presence of the deadly agent in cigarette smoke.

But the document also makes an assertion that experts say is highly misleading. The document declares flatly that the amount of nitrosamines in cigarette smoke had been reduced through filtration. That assertion was not in keeping with what was known about limitations of certain cigarette filters at the time, the experts say: smokers frequently compensated for them by inhaling more deeply, plugging up filter ventilation holes with their fingers or lips or taking more puffs.

The tobacco companies had been aware of this flaw in the filters for decades, according to industry documents and interviews, and Ms Gillibrand had just weeks before been briefed on their shortcomings and had taken a tour of the filtration section of Philip Morris’s production plant, according to company documents.

The presentation was given by Bill Dwyer, a scientist in the company’s research and development division, who described, among other things, how plugging the ventilation holes of filters diminishes the effectiveness of the filters.