Federal authorities on Saturday searched the home of James O’Keefe, the founder of the conservative group Project Veritas, according to witnesses and people briefed on the case, a day after Mr. O’Keefe acknowledged that the group was under investigation by the Justice Department in connection with a diary reported to have been stolen from Ashley Biden, President Biden’s daughter.

The F.B.I. carried out a court-ordered search of Mr. O’Keefe’s apartment in Mamaroneck, N.Y., early on Saturday morning, after having searched the homes of two associates of Mr. O’Keefe on Thursday as part of the investigation.

An F.B.I. spokesman on Saturday said that agents had “performed law enforcement activity” at the building, but would not discuss the investigation.

Mr. O’Keefe did not immediately respond to a request for comment on Saturday. But in a video statement on Friday, he said that his group had recently received a grand jury subpoena and acknowledged that Project Veritas had been involved in discussions with sources last year about the diary.

Jimmy Maynes, who lives next to Mr. O’Keefe at an apartment complex in Mamaroneck, said a handful of F.B.I. agents arrived early Saturday morning.

“They asked for James,” Mr. Mayne said, an entertainment manager. “I thought they were banging on my door. I opened the door.”

“They told me to close the door and I closed the door,” he added. “That’s exactly what happened. It was still dark.”

Brent Mickol, a teacher who lives across the hall from Mr. O’Keefe, said it was about 6 a.m. when agents arrived. Mr. Mickol said the agents said “something along the lines of ‘F.B.I. Warrant. Open up.’”

“I ran to the door and looked out the peep hole and clearly saw an F.B.I. raid,” he said. “You saw the jackets. Literally, it was just out of a movie.”

Mr. Maynes and Mr. Mickol said the F.B.I. agents were at the apartment for several hours.

In his video statement on Friday, Mr. O’Keefe offered a lengthy defense of his group’s handling of the diary, saying that he and his colleagues had been operating as ethical journalists, had turned the diary over the law enforcement authorities last year and had sought to return it to a lawyer for Ms. Biden.

“It appears the Southern District of New York now has journalists in their sights for the supposed crime of doing their jobs lawfully and honestly,” Mr. O’Keefe said in the video statement. “Our efforts were the stuff of responsible, ethical journalism and we are in no doubt that Project Veritas acted properly at each and every step.”

Project Veritas did not publish Ms. Biden’s diary, but dozens of handwritten pages from it were posted on a right-wing website last year a week and a half before Election Day, at a time when President Donald J. Trump was seeking to undermine Mr. Biden’s credibility by portraying his son Hunter as engaging in corrupt business dealings. The posting was largely ignored by other conservative outlets and the mainstream media.

The website said it had obtained the diary from a whistle-blower who worked for a media organization that refused to publish a story about it before the election. It claimed to know where the actual diary was located and that the whistle-blower had an audio recording of Ms. Biden admitting it was hers.

Ms. Biden, 40, is Mr. Biden’s youngest child. She has maintained a low profile and attracted far less attention than Hunter Biden, her half brother.

The Trump administration’s Justice Department, then led by Attorney General William P. Barr, opened an investigation into the matter shortly after a representative of the Biden family reported to federal authorities in October 2020 that several of Ms. Biden’s personal items had been stolen in a burglary, according to two people briefed on the matter.

Mr. O’Keefe said in the video that “tipsters” had reached out to Project Veritas in 2020 to alert them to the existence of the diary, saying that they had stayed in a room that Ms. Biden had recently been in. But Mr. O’Keefe said that his group could not authenticate the diary and made an “ethical” decision to not publish it.

He said that Project Veritas gave the diary to “law enforcement” and attempted to return it to a lawyer representing Ms. Biden, who he said “refused to authenticate it.” Mr. O’Keefe portrayed the investigation as politically motivated, questioning why the Justice Department under Ms. Biden’s father was pursuing the case.

In recent weeks, federal investigators have reached out to at least one person who worked for Project Veritas to question that person about the diary, one of the people briefed on the case said.

On Thursday, federal authorities searched the residence in Manhattan of Spencer Meads, a longtime Project Veritas operative and confidant of Mr. O’Keefe, and an apartment in Mamaroneck linked to another O’Keefe associate.

Project Veritas has a history of targeting Democratic congressional campaigns, labor groups, news media organizations and others. The group conducts sting operations using hidden cameras and fake identities. At one point, Project Veritas relied on a former British spy named Richard Seddon to help train its operatives, teaching them espionage tactics such as using deception to secure information from potential targets.

Flyover Media, the company that owns the website that published the pages from the diary, is registered to the same Sheridan, Wyo., address as Mr. Seddon’s company, Branch Six Consulting International. Mr. O’Keefe, the founder of Project Veritas, was once the president of a company that later registered at the same address.