The police say Mr. Smollett hired two brothers to carry out the assault and paid them $3,500. They have a copy of the check used to pay them, the police said. Also recovered, they said, were phone records that showed Mr. Smollett speaking to the brothers an hour before the incident took place, and then an hour after.

In a document prepared for the bail hearing, prosecutors said they had video of the brothers at the scene, text messages they shared with Mr. Smollett and their testimony as to how Mr. Smollett had recruited them for the plan. He even had them visit the scene of what investigators contend was the fake attack, a spot near his home, on an earlier night to prepare, prosecutors said.

But, the prosecutor’s document said, a video camera at the spot that Mr. Smollett had hoped would capture a phony attack was pointed in the wrong direction.

Superintendent Johnson declined to indicate why investigators now believe that Mr. Smollett had also played the chief role in mailing himself a threatening letter. The letter, which arrived a week before the reported assault, contained a white powder (crushed ibuprofen) and a sketch of what appeared to be a man being hanged and phrases, including “You will die.” The return address said “MAGA,” a reference to a slogan from the Trump campaign.

Mr. Johnson referred further comment about the letter to the F.B.I., which is investigating that part of the case. The agency declined to comment.

The actor, who surrendered to the authorities on Thursday morning, has insisted the attack occurred and that he has not done anything wrong. The felony disorderly conduct charge he faces carries a maximum sentence of three years in prison.