Investigators were working on Friday to determine why a man attacked a group of New York Police Department officers with a hatchet before they shot him dead.

Police obtained a warrant to search Zale Thompson’s computer for clues about Thursday’s daytime assault, which left one officer with a serious head injury. Thompson’s activity on social media indicated he was a convert to Islam and included rants about injustices in American society and oppression abroad but offered no clear evidence of any affiliation with terror groups, police said.

Security video and witness accounts appeared to leave no doubt that Thompson purposely targeted four rookie police officers who were on routine foot patrol in a busy commercial district. Moments before the attack, the bearded suspect was seen on a street corner crouching down to pull the hatchet out of backpack before he charged the officers and began swinging the hatchet with a two-handed grip, police said.

Asked on Thursday if the attack could be related to terrorism, Police Commissioner William Bratton didn’t rule it out. He cited the fatal shooting of a soldier in Canada earlier this week – what officials there have called a terror attack – as reason for concern.

“This early on, we really cannot say yes or no to that question,” Bratton said Thursday.

The attack occurred at about 2 p.m. while the officers were standing together posing for a passerby’s photo. Without a word, Thompson swung at an officer who blocked the blow with his arm, police said. Another officer was hit in the back of the head and fell to the ground.

As the 32-year-old suspect raised the hatchet again, the two uninjured officers drew their weapons and fired several rounds, police said. The bullets killed the assailant and wounded a female bystander, police said. A bloody hatchet, about 18 inches in length, was recovered.

Both the officer and the bystander remained hospitalized Friday.

(AP)