New York attack suspect Sayfullo Saipov left a note pledging allegiance to Isis in or near the truck he used to kill eight people an injure more than a dozen, law enforcement officials have told several US media outlets.
Officials have already deemed the assault by Saipov an act of terrorism.
The reported discovery that he offered fealty to a leading terrorist organisation helps fill out an emerging portrait of the attacker and his motive.
New York Governor Andrew Cuomo had told reporters that the attacker likely acted alone. Past terrorist attacks have involved assailants who offered their allegiance to organisations like Isis despite not being active members or recruited in coordinated plots.
“There’s no evidence to suggest a wider plot or a wider scheme”, Mr Cuomo said. “These are the actions of one individual meant to cause pain and harm and probably death”.
New York Police Commissioner James O’Neil told reporters that witnesses relayed hearing Saipov say “Allahu Akbar” as he exited his vehicle.
Donald Trump had already implicitly linked the attacker to Isis, saying in a tweet that “We must not allow Isis to return, or enter, our country after defeating them in the Middle East and elsewhere”.
Mr Trump has invoked the rise of Isis as a rationale for sharply curtailing admissions of refugees and other immigrants.
By using his truck as a weapon to mow people down, Saipov mimicked a tactic used during attacks in Berlin and Nice. Isis has encouraged its adherents to do so, part of its wider message encouraging attacks in
Western countries as the organisation's self-proclaimed caliphate in the Middle East has crumbled.
As an American-assisted campaign has pushed Isis from its former strongholds in Iraq and Syria, terrorism experts have warned the organisation could increasingly shift to attacks on Western cities.
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies