The dossier proceeded to rivet the U.S. political class, win credibility within the Federal Bureau of Investigation, cast a shadow over the first two years of the Trump presidency and cost millions of dollars for investigations and lawsuits, only to eventually be mostly discredited. One reason was where much of the dossier’s information came from—anything but Kremlin insiders.

Instead, a Wall Street Journal review found, many of the dossier’s key details originated with a few people gossiping after they had been brought together over a minor corporate publicity contract.

The dossier’s author, former British spy Christopher Steele, relied mainly on a Washington researcher to gather information. According to FBI notes of an interview with that researcher, Igor Danchenko, he said he wasn’t comfortable with the assignment and some of his sources were old friends—one a former schoolmate—whose information Mr. Steele exaggerated.