The F.B.I. declined to comment on the interview with Mr. Kadirov.
Some details about the pair’s friendship and Mr. Kadirov’s account to the F.B.I. emerged on Friday. (He spells his first name Mokhammadzokir, and not Mukhammadzoir, as the authorities initially rendered it.)
The pair met in Florida, where Mr. Kadirov moved after arriving in the United States in 2014. The person with knowledge of the interview said they did not know each other well at the time. Neighbors in the apartment complex where Mr. Kadirov lived with his family in Tampa said they often saw Mr. Kadirov and Mr. Saipov together there and at a nearby mosque.
They became closer after both moved to New Jersey, the person said. They drove for Uber, though Mr. Kadirov also worked odd jobs: as a sales manager at a flooring company and for a car company.
Mr. Kadirov noticed Mr. Saipov growing more and more short-tempered and agitated, the person said, even fighting with customers and people on the road.
Mr. Kadirov gave his friend books on Islam and urged him to listen to the teachings of Uzbek religious leaders. But Mr. Saipov “didn’t seem interested,” the person said, and asked questions that Mr. Kadirov felt showed an ignorance of Islam.
Rather, Mr. Saipov seemed more committed to the most conservative outward observances of the religion. He wore his beard long and wild, in a style rare among Uzbeks. And neighbors and friends said Mr. Saipov’s wife wore a niqab as part of a full-body covering that left only her eyes exposed. In Uzbekistan, such conservative clothing is nearly unheard-of.
In recent months, Mr. Saipov began talking about the possibility of moving back to Uzbekistan, the person said. Money was tight, Mr. Saipov told people, and the long-haul trucking business that had supported him since he arrived in the United States in 2010 was no longer as lucrative. He sold his truck and began taking odd jobs, as well as driving for Uber.
Mr. Saipov’s plan, the person said, was to wait until his wife obtained American citizenship and then move back to Uzbekistan, where they are both from.
Why he veered so violently from that plan is still unclear.
Like Mr. Saipov, Mr. Kadirov received a green card through the diversity visa program. Mr. Kadirov, who speaks Uzbek, Arabic, Russian and English, took classes in Islamic studies and the Quran at Al-Azhar University in Cairo from 2005 to 2012, the person said. His father is a diplomat and his mother is a doctor.
More recently, he had been taking continuing education classes in New Jersey.
In a separate development on Friday, John J. Miller, the New York Police Department’s deputy commissioner for intelligence and counterterrorism, said at a news conference that investigators were combing Mr. Saipov’s phone records and internet contacts. He said investigators had interviewed witnesses who said they had seen Mr. Saipov in the weeks before the attack with a rental truck like the one used in the attack. Mr. Miller said investigators had also canvassed for video to try to corroborate those accounts.
As people gathered on Friday for prayers at a mosque near where Mr. Saipov lived in Paterson, N.J., some in the Muslim community there were preparing for a steep escalation in threats since the attack.
The Islamic Center of Passaic County said it had received eight threatening phone calls on Wednesday and Thursday. Officials there notified the Paterson Police Department, which told the center it was investigating the threats.
“We haven’t seen anything of this magnitude,” said a woman who worked for several years at the center and declined to give her name out of fear for her safety.
Hasan Husein, an interpreter for the imam at the Masjid Omar Mosque in Paterson, which is next to the apartment where Mr. Saipov lived, said threatening callers contacted them this week, too. The mosque reported them to the police.
“Quite a few calls,” Mr. Husein said after Friday’s afternoon prayer. “Bad words and, ‘Go back to your country.’”
At the mosque, where attendance for afternoon prayers spilled into an outdoor plaza, Manny Simreen said several people had driven by this week and shouted insults, like “Get out of the country!”
Michael J. Whitaker, a spokesman for the F.B.I.’s regional office in Newark, said that the bureau was investigating multiple threats against mosques.
”We are aware of several threats against the Muslim community in New Jersey and take every threat seriously,” he said. “We continue to urge the public to please remain vigilant and report any and all suspicious activity to law enforcement immediately.”