“There were flaws in his writing and flaws in his life,” Mr. Renner, who plays Webb in the film, said in a phone interview. “But that doesn’t mean he was wrong, and it certainly doesn’t mean he deserved what he got.”

The film argues that the same reflexes in the newspaper business that hold others to account can become just as merciless when the guns are pointed inside the corral. Big news organization like The Los Angeles Times, The New York Times and The Washington Post tore the arms and legs off his work. Despite suggestions that their zeal was driven by professional jealousy, some of the journalists who re-reported the story said they had little choice, given the deep flaws. Tim Golden in The New York Times and others wrote that Mr. Webb overestimated his subjects’ ties to the contras as well as the amount of drugs sold and money that actually went to finance the war in Nicaragua.

But Mr. Webb had many supporters who suggested that he was right in the main. In retrospect, his broader suggestion that the C.I.A. knew or should have known that some of its allies were accused of being in the drug business remains unchallenged. The government’s casting of a blind eye while also fighting a war on drugs remains a shadowy part of American history.

Mr. Webb eventually wrote his own book, “Dark Alliance: The C.I.A., The Contras and the Crack Cocaine Explosion,” and Nick Schou, a journalist who covered significant parts of Webb’s downfall, wrote “Kill the Messenger: How the C.I.A.’s Crack Cocaine Controversy Destroyed Journalist Gary Webb.” Both books deeply inform the movie, making the argument that journalism more or less ate itself while the government mostly skipped away with its secret doings intact.

Mr. Webb was a talented investigative reporter who concentrated on local corruption when he worked at The Cleveland Plain Dealer and then The San Jose Mercury News. When he was first approached about C.I.A. duplicity, he was deeply skeptical. But when the tipster, the girlfriend of a drug dealer on trial, said her boyfriend had ties to the C.I.A., she had enough evidence to convince him to read that 1988 report from a special Senate subcommittee documenting instances in which drug dealing by crucial allies, including some in Nicaragua, was tolerated in the name of national security. Major news outlets gave scant attention to the report.