In 1991, Mr. Horowitz joined the federal prosecutor’s office in Manhattan, where he briefly overlapped with Mr. Comey. He led a huge investigation that implicated almost an entire police precinct in Harlem in a scandal involving drug dealing, robbery and bribery.
“It took an extraordinary amount of independence,” said Mary Jo White, the former United States attorney in Manhattan. “Prosecutors work with police on their cases, so he was investigating and prosecuting, in effect, his partners in law enforcement.” He won an award from the attorney general, and Ms. White named him head of her office’s public corruption section.
In 1999, Mr. Horowitz moved to Washington to join the Justice Department’s criminal division and became chief of staff to its leader, James K. Robinson, where his judgment was lauded. “He had to know what to ask the prosecutors, without having intimate knowledge of a case, judge the quality of the incoming information and send it to the boss,” said Shan Wu, who was counsel to Attorney General Janet Reno, noting that Mr. Horowitz took notes on a single notecard and simply remembered the rest of the information.
Mr. Horowitz left the Justice Department in 2002 to join Cadwalader, Wickersham & Taft as a defense lawyer and was later nominated by President George W. Bush to the United States Sentencing Commission. In 2012, President Barack Obama tapped him to be the Justice Department’s inspector general.
By the time Mr. Horowitz started his job, his office was well into its investigation of the Fast and Furious scandal, a botched operation in which officials from the Bureau of Alcohol, Tobacco, Firearms and Explosives let guns enter Mexico in the hopes of tracing them back to criminals.
Mr. Horowitz handled the politically fraught situation by reaching out to Democrats and Republicans on the congressional oversight committees. In a meeting with former Representative Jason Chaffetz, Republican of Utah, and Representative Trey Gowdy, Republican of South Carolina, Mr. Horowitz told them about his own conflicts of interest and said he had assigned others in his office to handle those parts of the investigation.