Mr. Hickey said that he stayed awake at night, feeling paranoid. He was hospitalized in a stress-induced delirium, and was screaming, biting and spitting, according to medical records shown at trial.

Four days after his release from the hospital in October 2015, he received a grand jury subpoena, he said.

Mr. Hickey decided to plead guilty to his role in the conspiracy and cooperated with prosecutors as their star witness, testifying on the stand for three days.

At trial, lawyers for Mr. McPartland and Mr. Spota tried to shred Mr. Hickey’s credibility, calling him a practiced liar and highlighting that he admitted to four extramarital affairs. They pointed to a state judge’s determination in 1990 that Mr. Hickey had lied under oath as a police officer in a different burglary case.

The defense said prosecutors were relying on Mr. Hickey’s recollection of conversations that happened years ago, with no concrete evidence to corroborate his memory. No other witness testified to receiving direct orders from Mr. Spota or Mr. McPartland to obstruct the investigation, defense lawyers argued.

Prosecutors produced calendar entries and call records that they said showed Mr. Hickey was telling the truth.

Ultimately, the 12 jurors chose to believe Mr. Hickey.

On Tuesday, after the verdict was read, Mr. Loeb, the man who was assaulted by the police chief, poked his head into the emptying courtroom. He had been watching the trial from the overflow room. He was smiling.