Police believe an Uzbek immigrant suspected of killing eight people in New York city planned his attack for weeks using instructions circulated by ISIL.
Sayfullo Saipov, 29, was shot and then arrested on Monday after a rented pickup truck raced along a bicycle path in Lower Manhattan hitting pedestrians and cyclists.
The chaos erupted just five blocks from the site of the old World Trade Centre towers and represented the deadliest attack in the city since 9/11.
The dead included five Argentinian men who travelled to the city for a 30-year school reunion, two Americans and a woman believed to be either German or Belgian.
Police said they had interviewed the suspect despite his serious wounds and believe he acted alone.
“It appears that Mr Saipov had been planning this for a number of weeks,” said John Miller, deputy police commissioner.
He said handwritten notes in Arabic found in the vehicle proclaimed that ISIL “would endure forever”.
“He did this in the name of ISIL, and along with other items recovered at the scene were some notes that indicate that," said Mr Miller. "He appears to have followed almost exactly the instructions that ISIL has put out on its social media channels to its followers."
He added that Saipov had never been the subject of a New York police investigation but appeared to have links to other people who had been investigated.
Officers declined to comment on reports that Saipov was boasting about the attack in hospital, where he underwent surgery after the attack.
President Donald Trump described the suspect as an “animal”. He promised tighter restrictions on people entering the country and said he would terminate the Diversity Visa Programme, better known as the “green card lottery”, which he said was how Saipov arrived in the country.
“We will take all necessary steps to protect our people,” Mr Trump added.
Officers searched the suspect’s home in Paterson, New Jersey, where he was believed to live with his wife and two children, as they tried to piece together his life in the US after arriving in 2010.
A friend and fellow Uzbek national, who knew him when he lived in Florida, said there was no sign he would carry out such an attack.
Kobiljon Matkarov said, "He was very happy guy. He liked the US. He is no terrorist. He's all the time happy, smiling all the time.”
He added that Saipov, who has a commercial truck licence, was working as an Uber driver.
Officers have begun constructing a timeline of the attack. They believe he rented the vehicle from Home Depot in Passaic, New Jersey, at about 2 pm before driving into Manhattan.
Witnesses said they saw the pickup careering at speeds of up to 65 kph down the West Side Highway bike path shortly after 3pm.
The vehicle came to a halt only after it collided with a school bus. Witnesses told police that Saipov shouted “Allahu Akbar” as he leapt from the vehicle, wielding what appeared to be two guns. His rampage finally ended when he was shot in the abdomen by a police officer.
City officials immediately praised the quick thinking of Ryan Nash, 28, a five-year veteran of the force who happened to be responding to an unrelated call.
“He was a hero,” said Andrew Cuomo, the governor of New York. “I think Officer Nash showed how important (patrol officers) are, and how talented they are and how brave. So we all applaud and congratulate him.”
The two weapons turned out to be a BB gun (a type of air gun which fires spherical projectiles similar to shot pellets) and a paintball gun.
No group has yet claimed involvement in the attack. However, both ISIL and Al Qaeda have encouraged followers in the West to turn vehicles into weapons.
The November issue of Rumiyah, a magazine published by ISIL, offered detailed instructions for mounting such an attack, including recommending that a gunman leaps from the vehicle to launch a second phase of killing, and offering suggestions on how best to ensure the movement gets full credit.
“An example of such would be simply writing on dozens of sheets of paper “The Islamic State will remain!” or “I am a soldier of the Islamic State!” prior, and launching them from the vehicle’s window during the execution of the attack,” the magazine recommends.
New Yorkers woke to a heavy police presence outside the World Trade Centre and at other key locations around the city, which is gearing up for the New York City Marathon on Sunday, when as many as 2.5 million people are expected to cheer on more than 50,000 runners.
Officials said they had no intelligence of further imminent plots but said extra security would be provided.
Mayor Bill de Blasio, said terrorists could never change New York’s status as an international, global city.
“We will not be cowed. We will not be thrown off by anything,” he said, before remembering the six foreigners who died.
“They saw New York as a special place to be,” he said. “We now and forever will consider them New Yorkers.