In certain precincts of establishment Washington, there are few people more familiar than Samuel R. "Sandy" Berger. For three decades, the former Clinton national security adviser has stood at the intersection of politics and foreign policy, making himself well-known and for the most part well-liked on Capitol Hill, in think tanks and among Democratic campaign operatives, as well as White House and foreign policy reporters.

This familiar figure now is the subject of mystery, even among colleagues who know him best: How could something like this happen to someone like him? Sound judgment and attention to detail, supporters say, have been signatures of Berger's steadily ascending career. These traits, by his own admission, were notably absent in his improper handling of classified documents from the National Archives.

"Hardworking beyond belief," recalled former deputy secretary of state Strobe Talbott, describing Berger as an "immensely judicious . . . hardheaded public servant."

"He's a tremendously hands-on person, and not somebody who takes problems and turns to somebody and says, 'You handle it,' " said James B. Steinberg, who served as Berger's deputy at the National Security Council from 1997 to 2000.

The questions of how and why Berger left the Archives building with copies of documents that were supposed to remain there, subsequently losing some of them, is under criminal investigation. Already, the controversy forced Berger's departure as an informal adviser to Sen. John F. Kerry (Mass.) and interrupted a cycle he has followed for decades. In presidential elections, he is a foreign policy adviser to Democratic candidates. When they win, he goes into power with them; when they lose, he adjourns to a lucrative and well-connected legal and international consulting business. He met Bill Clinton in 1972, when they were young activists working for presidential candidate George S. McGovern.

At one end of the spectrum lies Berger's explanation for the documents: Through simple carelessness, on two occasions, he inadvertently scooped up papers he meant to leave behind after a long day perusing records at the Archives. At the other end are the loudly voiced suspicions of Republicans: that he deliberately took documents to improperly share information or to hinder the Sept. 11 commission's investigation.

In between these possibilities is another, which even some former Berger colleagues say strikes them as plausible. As a busy man who has held one of the most influential positions in government and who sometimes projects a harried and impatient demeanor, Berger might well have bridled at the restrictions at the Archives, associates acknowledge. Government sources have said Archives employees reported that Berger acted imperiously on some visits and sometimes asked to be left alone so he could make private calls. Eager to give certain materials a sustained reading, not eager to sit indefinitely watched by minders like a student in study hall, Berger may have concluded there was no harm in taking copies of a Clinton-era report on terrorism that existed only because he had ordered it written, some former colleagues believe.

Berger's attorneys have acknowledged that something approximately like this took place with 40 to 50 pages of handwritten notes. He was supposed to allow Archives officials to review his notes before leaving but regarded this rule as a minor matter and did not comply, lawyers said. One former Clinton colleague of Berger's, who did not speak for the record because of the sensitivity of the criminal investigation, suggested that Berger knew he was "cutting corners" but not in a consequential way -- a bit like the person who runs a red light at 3 a.m.

But his attorneys say that conscious corner-cutting is not what happened with draft copies of an "after-action report" that Berger ordered prepared to review the government's response to terrorist plots at the turning of the millennium. The report outlined steps the government needed to take to improve safety, some of which were taken and some of which were not.

Berger did not mean to take copies of earlier versions of the report with him during Archives visits in September and October, said his attorney Lanny Breuer, and realized that he had done so only when Archives officials alerted him on Oct. 4, two days after his last visit. He returned the documents he could find -- some remain missing -- the next day, and he turned over his handwritten notes a few days after that.

Skeptics have questioned how Berger could inadvertently take copies of the same document on two occasions and then lose some of the papers. Defenders -- including Clinton, in comments to the Denver Post -- said people who have seen the mountain of files on Berger's desk would find it easier to believe.

For now, the question is what impact the revelation of Berger's lapse, and the uncertain outcome of the investigation, will have on the career of someone who is an emblematic figure of the Democratic government-in-waiting.

The controversy has touched on Berger's two preoccupations of recent years. While running a new consulting business, he has had his gaze fixed partly on the future -- and evicting President Bush from power -- and partly on the past. Since Sept. 11, 2001, Berger has been the most ardent defender of the Clinton record in combating terrorism. He has argued that Clinton responded aggressively to the rising al Qaeda threat, given the political and diplomatic circumstances that existed before the World Trade Center and Pentagon attacks.

Berger is declining interview requests while the matter is under investigation. He was regarded as a strong contender to be secretary of state if Kerry defeats Bush, but many Democrats say that prospect has dimmed considerably in the light of recent news.

John D. Podesta, who served as Clinton's chief of staff during part of Berger's White House tenure, predicted Berger will have more resilience than some Washington figures who run afoul of controversy. "The strength of Sandy's career has not been connections and influence-peddling, it's been intellect and ability to have a penetrating analysis about foreign policy," he said.

The criminal probe of papers taken from the Archives puts Sandy Berger's future in Washington on the line.