ith his reports in 1969 of a massacre by young American soldiers at My Lai, Seymour M. Hersh began his run as one of journalism's best-known investigative reporters. But in spite of his longevity, none of his subsequent reportorial efforts has had the impact of his first. Until now.

In less than a month, Mr. Hersh, 67, has written three articles in The New Yorker that have helped set the political agenda by reporting that once again American soldiers in the midst of a war where the enemy is elusive and the cause is complicated had committed atrocious acts.

The two sets of articles, on My Lai and on the mistreatment of Iraqis at the Abu Ghraib prison, serve as bookends on a career built on dogged pursuit. And longtime associates of Mr. Hersh say it is no coincidence that the news development that has brought him back to the center of American journalism is one in which right and wrong are separated by bright, clear lines.

Throughout his career Mr. Hersh has served as a sort of a global cop reporter, working disaffected bureaucrats, intelligence operatives and soldiers to uncover intelligence pratfalls, foreign intrigues and administration wrongdoing.

"There is a sense of outrage at this story for him," said Tom Goldstein, a professor of business and journalism at Arizona State University who was a colleague of Mr. Hersh at The New York Times (and his tenant) in the 1970's. "He has a sense of outrage unlike anyone I have ever known, and it remains undiminished over a career."

The subjects of his reporting often find themselves outraged as well. On Monday the Defense Department accused him of writing a "hysterical piece of journalistic malpractice" for this week's article in The New Yorker reporting that Secretary of Defense Donald H. Rumsfeld approved the use of a clandestine unit to find terrorists within the walls of Abu Ghraib prison. In Bob Woodward's "Bush at War," the president called Mr. Hersh a liar.

And Gen. Barry McCaffrey, who was accused by Mr. Hersh in a lengthy New Yorker article in 2000 of authorizing the massacre of fleeing Iraqi troops at the end of the first Gulf War, said the charges were outrageous. "The guy just fundamentally lacks integrity," he said in an interview.

Mr. Hersh and the editor at The New Yorker, David Remnick, stand by his reporting, and many former colleagues and press critics praise Mr. Hersh's career-long body of work.

Unlike his colleagues at newspapers or on television, Mr. Hersh can be quite subjective in his judgments; anyone who is reading his current magazine articles is well aware he is against the war. To Mr. Hersh, what took place at Abu Ghraib ó and the responsibility that higher-ups bear for the abuses ó represent a grievous perversion of the American mission.

"My Lai and Vietnam was a technical problem," he said. "America was not jeopardized. This story represents a very important strategic loss, not something that can be fixed by setting up an embassy and giving people some breaks on trade."

Because Mr. Hersh mines intelligence channels, his articles often depend on anonymous sourcing. The lack of names, which Mr. Hersh has said is born of necessity, leaves him open to attack. (Mr. Remnick said he knew the identities and agendas of the anonymous sources.)

This week's claim that Mr. Rumsfeld himself ordered more aggressive treatment of prisoners has yet to be duplicated.

"All of the dots aren't connected yet," said Jeff Leen, assistant managing editor for investigations at The Washington Post. But he is not one to bet against Mr. Hersh. "He is a legend and deservedly so."

Jack Shafer, press critic for Slate, an online magazine, said: "He has gotten lots of stuff wrong or only half right. He predicted that C.I.A. director George Tenet was done, that special ops and air power could not defeat the Taliban, he said that the war in Iraq had faltered and we were in danger of stalemate. Ten days later, Baghdad fell. But after having a bad early war, he has been on a blistering run."

Mr. Hersh won a National Magazine Award last month for his articles last year about corruption within the Saudi royal family and stumbles in the war on terror by the United States military and intelligence.

"He is the consummate muckracker," Mr. Shafer said. "It's just important to catalog his hits and misses."

Mr. Hersh got his start in journalism during the 1960's at the City News Bureau in Chicago, where he wrote numerous articles about crime. After stints at United Press International and The Associated Press, he made a brief detour as a press secretary to the presidential candidate Eugene McCarthy. He soon returned to reporting, and came across the events at My Lai.

He received the Pulitzer Prize in 1970 and was eventually hired by The Times to help the newspaper catch up with The Post on the Watergate scandal. He published a number of influential articles at the paper, including breaking the news of broad-based domestic spying by the C.I.A., but it was a fitful collaboration.

He went on to write a number of books, including a damning portrait of Henry A. Kissinger, "The Price of Power," published in 1983. He returned to The Times for several special projects, but eventually landed at The New Yorker. Mr. Hersh's journalistic crusades have received mixed reviews over the years. For example, his assertion in his book "The Samson Option," about the Israeli effort to build a nuclear bomb, that there was a political conspiracy in the United States to delay the release of the American hostages in Iran and prevent President Jimmy Carter's re-election caused an uproar and suggestions that he had overreached.

Mr. Hersh is often compared to Mr. Woodward, an occasional antagonist with whom he competed on Watergate. Mr. Woodward, an assistant managing editor for The Post, has also driven the news agenda recently, with his latest book, "Plan of Attack" (Simon & Schuster). "They are like Babe Ruth and Ty Cobb," said Mr. Leen of The Post. But while Mr. Woodward is on a first-name basis with many of the administration's highest ranking officials, Mr. Hersh sticks to the back channels for articles that often countervail the official wisdom.

Mr. Hersh is a noir version of Mr. Woodward, darker and brusquer. Robert Baer, a former C.I.A. officer and a friend and source of Mr. Hersh, can recall being dragged out of bed on a Sunday morning at 7 a.m. to go to breakfast.

"He calls me and 10 other military and intelligence guys by 8 in the morning every day," said Mr. Baer, who has a Labrador that he has affectionately named Hersh who is fond of digging through trash.

That intensity is on display in his working relationship with Mr. Remnick at The New Yorker. "Has there been any screaming and yelling along the way? Sure," Mr. Remnick said. "Like any good marriage we have had our moments."

Last week Mr. Hersh called Mr. Remnick at a hotel in California at 4 in the morning. "There was no `Hello,' no `How are you?' " Mr. Remnick said. "Just the news that he had nailed down a source, and then he was gone."