NEW YORK -- New York City police Commissioner William Bratton says the hatchet attack on four rookie officers was a terrorist act by a homegrown radical.

Bratton said Friday the suspect, Zale Thompson, was a Muslim convert who ranted online against America, but had no clear ties to international terrorism. He believes Thompson was self-radicalized.

Zale Thompson. NYPD

Thompson was killed by police. One officer is hospitalized with a head wound.

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Bratton says investigators are trying to determine whether the attack was planned.

Police are examining Thompson's computer for clues. Bratton says investigators found that Thompson browsed for organized terror groups, as well as beheadings and the shooting in Canada earlier this week.

Authorities also are trying to determine if Thompson had any history of mental illness. But Bratton says he is comfortable calling it a "terrorist attack."

CBS New York reports that a bloody hatchet, about 18 inches in length, was recovered.

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Bratton and other experts have been warning for months about the possibility of lone wolf terror attacks.

A Facebook page linked to Thompson includes a picture of an Islamic warrior and a quote from the Koran.

On YouTube, Thompson appears to have posted a comment advocating "holy war," saying "If you're looking for 'perfect' Muslims who never make any mistakes in their jihad, then you will be looking in vain," CBS 2's Tony Aiello reported.