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A broad-daylight hatchet assault on a group of New York police officers -- leaving one critically injured with a head wound -- was carried out by a self-radicalized convert to Islam in what officials Friday termed a terrorist attack, reports CNN .

Officer Kenneth Healey, who remains in critical condition, and three of his on-duty colleagues were posing for a picture on the sidewalk Thursday afternoon when a hatchet-wielding man charged at them "unprovoked," according to Police Commissioner Bill Bratton.

The attacker, identified as Zale H. Thompson, was hiding behind a bus shelter as if he was waiting to attack the officers, according to a law enforcement official, who said it almost appeared as if he were stalking them.

"This was a terrorist act," Bratton said Friday.

In the aftermath, some observers questioned if the attack was tied to recent calls by radicals to strike the military and police, especially on the heels of separate attacks on officers in Canada earlier this week.