Neighbors in Fort Greene, Brooklyn, said Mr. Thompson was a bright but troubled young man whose behavior in recent months had grown odd.

“His behavior, that young man, let you know that something is wrong with him,” said Sherman Nelson, 72, Mr. Thompson’s neighbor in Brooklyn. He had often seen Mr. Thompson hopping subway turnstiles at a station in Brooklyn.

Mr. Thompson lived with his mother and brother, Mr. Nelson said. When he was younger neighbors said they saw him boarding a bus for children with learning disabilities.

Then, Mr. Nelson said, the young man’s behavior started to change.

John Thompson, a relative, also said Isaiah Thompson’s behavior had changed “out of the blue.”

“He’s troubled and he needs some help,” said John Thompson. “He’s a good kid, he’s just troubled.”

The public hunt for the subway brake puller began earlier this week, when the Metropolitan Transportation Authority brought a pattern of emergency brake incidents to the police department. After reports of cascading rush-hour delays on the 2 and 3 lines in Manhattan on Tuesday night, the M.T.A. believed it was the work of a repeat offender.

There had been internal reports going back months noting that a man had gained access to the controller’s chair at the rear of a train and had pulled the emergency brake. Officials initially feared that the culprit had obtained a key that allowed him to access controller cabins from the outside of trains.

Confronted by officers early Friday, Mr. Thompson admitted that he had previously pulled subway emergency brakes, the police said. He did not remember the two specific incidents in which he was charged, and denied having a key to train compartments.