Court filings by defense lawyers for Sayfullo Saipov, the man who used a rented truck to kill eight people on a Manhattan bike path on Halloween 2017, indicate the perpetrator was known to the F.B.I. and caught on a wiretap the day before he carried out his deadly attack.

The New York Times reported on Monday that the filings did not include transcripts of the conversations or indicate whether Saipov made any threatening comments on the recording.

Defense lawyers said they obtained materials from the F.B.I. that included “numerous audio recordings of conversations between Mr. Saipov and various targets of F.B.I. surveillance.” Most of these conversations were apparently conducted in the Uzbek language.

The defense team said prosecutors told them that they would not use the recorded conversations in court against Saipov, but they noted F.B.I. interviews with Saipov included questions about “matters and contacts that overlap” with the covert surveillance.

The Times quoted legal and security experts troubled by the discovery that the perpetrator of the deadliest terrorist attack in New York since 9/11 was on the federal radar screen:

“It’s possible that the picture they paint of Saipov is just not that alarming,” said Bobby Chesney, a professor at the University of Texas School of Law and co-founder of Lawfare, a national security blog. “But it could turn out otherwise, and that would raise serious questions about why they didn’t make him a priority, including the possibility of arresting him if grounds existed to do so.” Karen J. Greenberg, director of the Center on National Security at Fordham University School of Law, said, “The question it raises in my mind is no matter how much surveillance there is, even right up to the eve of an attack, we still have not figured out exactly what it is that we’re looking for as a sign of immediate danger beyond ‘I have a gun in my pocket’ or ‘a bomb strapped to my chest.’”

Prosecutors are seeking the death penalty for the 30-year-old Saipov, who said he was inspired by Islamic State videos to carry out his carefully-planned vehicular rampage and feels “good” about the results he achieved.

Saipov is so unrepentant that he asked permission to fly an ISIS flag outside his hospital room while recovering from the two gunshot wounds inflicted during his capture. At his trial in June, he told the court its ruling matters less to him than “Allah’s judgments” and he remains committed to the Islamic State’s war to “impose sharia [Islamic law] on Earth.”

Defense lawyers asked the judge to block the death penalty because President Donald Trump called for it on Twitter soon after the Halloween attack and “politicized the decision.”