Authorities then announced Mr Kadirov had been "located" but did not offer any further information about where he was found, or details of his link with Tuesday's attack.
"We have found him and I'll leave it at that".
The FBI has not released what Kadirov's supposed connection to the attack may be.
The person in touch with Kadirov's family says the two men knew each other only from their days as fellow drivers for the ride-sharing app Uber and were not close friends. According to NBC, it's not clear if Kadirov was aware of Saipov's terror plot, but authorities have said "there is nothing that indicates he may have been involved in the attack".
At a midday news conference, police officials said Saipov was never the subject of an NYPD investigation but said he might have had contact with individuals who were. The person said that the group encouraged Kadirov, who lives in New Jersey, to cooperate with authorities, and that the man has done so.
Both Saipov and Kadirov are from Uzbekistan, a country in Central Asia that formerly was part of the Soviet Union.
The discovery came after Saipov, a 29-year-old Uzbek, was hit with federal terrorism charges for allegedly plowing a truck down a lower Manhattan bike path Tuesday afternoon.
Saipov was charged Wednesday with terrorism crimes.