The Times, citing two unnamed law enforcement officials briefed on the inquiry, said investigators have zeroed in on an individual seen in video footage of the riot who attacked several officers with bear spray, including Brian Sicknick, the policeman who died.
Sicknick, 42, was among a vastly outnumbered group of police officers confronted by the mob who stormed the Capitol in a bid to stop Congress from certifying the election of President Joe Biden.
The violence led to the impeachment of Trump by the US House of Representatives on a charge of inciting an insurrection, but he was acquitted by the Senate in a trial held after he left office.
According to the New York Times, FBI agents began to suspect soon after opening a homicide probe that Sicknick's death was related to his inhalation of a chemical irritant, such as mace or bear spray, which both law enforcement officers and rioters were armed with during the insurrection.
According to one of the officials cited in the Times' report, video evidence shows that Sicknick's suspected assailant discussed attacking officers with bear spray beforehand.
Medical examiners have yet to rule on the cause or manner of Sicknick's death, as the autopsy is pending results of toxicology tests, the Capitol Police said in a statement on Friday.
Well over 100 officers were injured in the riot and five people died.
Although investigators have narrowed potential suspects seen in video footage to a single person this week, they have yet to identify that individual by name, the Times reported.
The newspaper said the US Justice Department declined to comment. The Federal Bureau of Investigation declined to comment to Reuters.More than 200 people have been arrested for their role in the Capitol siege, a number of them associated with militant groups such as the Oath Keepers and Proud Boys, underscoring rising concern about threats posed by right-wing extremists.