Footage recorded by a bystander appears to show Saipov brandishing a gun as he weaves through traffic just moments after the incident.
Officials later said he was carrying pellet and paintball guns. He was apprehended by police after being shot in the stomach, but is expected to survive.
Police said they were treating the incident as a "terrorist attack" and the FBI has taken charge of the investigation.
Nobody has made a claim of responsibility for the incident and Isis’s evening “news bulletin” did not mention the attack.
Saipov may have left a note pledging allegiance to Isis in or near the truck, law enforcement officials told several US media outlets, however this detail has not been confirmed.
New York Governor Andrew Cuomo told reporters the attacker is likely to have 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 witnesses relayed hearing Saipov say "Allahu Akbar" as he exited his vehicle.
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