A hatchet attack on New York rookie police officers was a “terrorist act” carried out by a self-radicalised, 32-year-old loner who recently converted to Islam, police said Friday.

“This was a terrorist act,” police commissioner Bill Bratton told a news conference one day after the attack, saying he was “very comfortable” describing the assault as a “terrorist attack”.

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

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

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.”

He had no police record in New York but had been 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.