New York (AFP) - A hatchet attack on New York police officers was a "terrorist act" carried out by a self-radicalized Muslim convert who had been in the military and browsed Al-Qaeda websites, police said Friday.

"This was a terrorist act," police commissioner Bill Bratton told a news conference on Friday, one day after the attack, saying he was "very comfortable" describing it as a "terrorist attack."

Police said Zale Thompson, who was 32, unmarried and unemployed, appeared to have acted alone and was not affiliated to a particular group, but that the investigation was ongoing.

A loner who spent hours locked away in his bedroom, he had looked at websites about groups such as Al-Qaeda and Islamic State, and watched beheadings and Wednesday's deadly attack in Canada.

Officer Kenneth Healy, 25, is in hospital in a critical but stable condition after being injured in the back of the head during Thursday's broad daylight attack in a busy shopping area.

Another officer was hit in the arm in the assault in New York's borough of Queens. The group of four young police officers had graduated from the police academy only months before.

In an attack that lasted just seven seconds, Bratton said Thompson charged with a hatchet in his hand, striking two officers before he was shot dead by the two other officers, who were uninjured.

A graphic video of the attack has been released, showing a bearded Thompson dressed in a green jacket, running towards his victims and swinging the hatchet in both hands.

A 29-year-old female bystander was accidentally shot and is also in hospital in a critical but stable condition, Bratton said.

Police said Thompson converted to Islam two years ago and that relatives described him as a "recluse" and "lately depressed."

An axe and a large hunting knife were recovered from his home and Thompson made anti-Western, anti-government and in some cases anti-white statements on social media, police said.

He visited websites that focused on terror groups such as Al-Qaeda, the IS organization and the Shebab Islamists in Somalia.

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Police said Thompson's Internet browsing history included the fence-jumping incident at the White House this week and Wednesday's shooting in Canada.





- Online history -

"It appears... this is something he has been thinking about for some time and thinking about with more intensity in recent days," chief of detectives Robert Boyce said.

Police believe that Thompson acted alone and was self-directed.

"The investigation is hoping to determine as quickly as possible if there were any other actions that he was engaged in with others that might indicate a continuing a threat," Bratton said.

Police said they were investigating whether Thompson was affiliated with any mosque or association, but said most of his activity and exposure appears to have been through the Internet.

"The father described that he spent extensive amounts of time by himself in his bedroom and by all accounts was a true proverbial loner," said Bratton.

Thompson had no police record in New York but had come into contact with the force as a victim of assault when he was 16, and was arrested six times in California in 2003-04.

He spent three years in the military but was involuntarily discharged in 2003, most likely due to drugs, police said.

SITE, a private terrorism monitoring group, said that Thompson displayed "extremist leanings" in an array of statements on YouTube and Facebook.

Bratton said the issue of a lone wolf, self-radicalized assailant was one of "increasing concern" to counter-terrorism officers.

SITE said Thompson described "jihad as a justifiable response to the oppression of the 'Zionists and the Crusaders'" in a comment posted to a pro-Islamic State video on September 13.

Queens residents said that they were disturbed by the attack.

"A thing like that isn't supposed to happen, children can be on the streets," said a woman who gave her name as Helena.

"Anyone could get caught in this incident and I don't think that's right, it's not right," she told AFP.