Sixteen people in his office died, and if he no longer thinks about it every day, he does worry that the ideology behind the attack has found new adherents.

“That awful underbelly of American life that has been there for a long time is still there and it is still dangerous,” he said. He noted that members of the far right, white supremacists and neo-Nazis like those who marched in Charlottesville, Va., in August 2017 continue to spout the same anti-government, anti-Semitic rhetoric. “Ideologies are sometimes not just ideologies. They result in awful actions and we need to be vigilant about that.”

When the F.B.I. traced the Ryder rental used as the bomb, they discovered Mr. McVeigh jailed in a small Oklahoma town. He had been detained by a vigilant state trooper because his getaway car had no license plate and no documents and he was wearing an unlicensed Glock pistol.

Terry L. Nichols, now 65, an Army buddy of Mr. McVeigh, was sentenced to life in prison as an accomplice and Michael J. Fortier, who knew about the plot but did not report it, was released from prison in 2006 after serving more than 10 years of a 12-year sentence. He and his wife, Lori, were cooperating witnesses in both trials.

There is a continued debate whether the F.B.I. traced everyone connected to the plot. Mr. McVeigh maintained that he acted largely alone, a model of the “leaderless resistance” strategy that white supremacists adopted in the 1980s.

To some extent, law enforcement agencies perpetuate a falsehood when describing attackers like Mr. McVeigh as “lone wolves,” said Kathleen Belew, a University of Chicago historian and the author of “Bring the War Home: The White Power Movement and Paramilitary America.”

That discounts the far-right ecosystem both online and on the ground that nourishes such killers. “We can see that they are quoting the same texts, that they have all these social ties with one another, that they are working for the same objectives,” Dr. Belew said. “As long as society buys into this fiction of the lone wolf, there is no movement to confront.”