The motive for the killing was unclear, Dermot F. Shea, the Police Department’s chief of detectives, said at a news conference.

“Was he acting alone?” he said. “Was he acting for other people? What was the motive? I simply, standing here in front of you, do not have all the answers.”

He identified the man as Anthony Comello, and sources said he was taken into custody in Brick Township, New Jersey, on Saturday morning. He was being held in the Ocean County Jail at the request of the Police Department, according to a jail official.

Several officials said preliminary information suggested that the killing of Francesco Cali, who was shot outside his home on Staten Island on Wednesday night, was not related to organized crime. However, one official cautioned that the inquiry was still in its early stages.

After speaking with detectives Saturday morning, Comello retained a lawyer and has not given a statement since.

“We expect him to be charged with the murder of Francesco Cali,” Shea said.

Two law enforcement officials said Comello had no criminal history, but had nonetheless previously come to the authorities’ attention.

Sometime in recent years, his strange behavior in a federal courthouse was such that the U.S. Marshals Service briefly took him into custody and asked the New York Police Department’s Intelligence Division to check its records to determine if he had any history of terroristic threats. Neither of the officials had any details about the specific nature of his behavior, but both said no such history could be found.

Fingerprints were recovered from Cali’s vehicle, but police would not confirm that the forensic evidence pointed toward Comello. Police said they have not recovered a weapon.

Cali, 53, was shot 10 times, the chief said. Police said they received a report about the shooting outside 25 Hilltop Terrace in the Todt Hill section of Staten Island around 9:20 p.m.

Around 9:17 p.m., a pickup truck driven by Comello backed into Cali’s parked Cadillac Escalade, damaging it. He then walked up to the door of Cali’s house and rang the bell, where, law enforcement sources said, his face was caught on video.

Cali and Comello spoke for roughly a minute, in a conversation also recorded by surveillance cameras and reviewed by investigators. The footage showed Comello, dressed in a hooded sweatshirt and a baseball cap, bend down and pick up a license plate that had fallen off the Escalade.

Cali took the license plate and walked back to his car, where he placed in it the rear. At that moment, the gunman pulled out a 9 mm pistol and fired 12 times, officials said.

The truck Comello drove was recovered Saturday morning in New Jersey. The handgun used to kill Cali has not been found.