David E. Patton, the chief federal public defender in New York City, who was representing Mr. Saipov, asked that he receive a daily change of dressing on the wounds he sustained after being shot by a police officer.

“He is in a significant amount of pain,” Mr. Patton said.

The grievous injuries to victims, the scope of the inquiry and Mr. Saipov’s path toward extremism all began coming into view on Wednesday. The F.B.I., after saying it was trying to learn more about a second Uzbek man in connection with the attack, later announced that investigators had found the man, Mukhammadzoir Kadirov, 32, in New Jersey. It was not clear why federal authorities wanted to question him in connection with the attack.

The authorities questioned Mr. Saipov after he waived his Miranda rights at a Manhattan hospital, the complaint says. They were also questioning Mr. Saipov’s wife, Nozima Odilova, who was cooperating, law enforcement officials said. The couple live in Paterson, N.J., and have three children.

As investigators looked into whether Mr. Saipov’s Uzbek contacts may have handed him off to an ISIS operative, they pieced together parts of his past, law enforcement officials said. He attended a wedding in Florida of an Uzbek man who was under scrutiny by the F.B.I. But his attendance didn’t trigger a separate investigation of him, the officials said.

Image Mukhammadzoir Kadirov Credit... Department of Justice

Investigators were still looking into whether Mr. Saipov had links to other federal counterterrorism inquiries.