New York City police are calling a hatchet-wielding man's attack on a group of police officers in broad daylight on Thursday an act of terrorism carried out by a radicalized Muslim convert, USA Today reported. A clear motive, however, still needs to be established in the case, police confirmed.

Zale Thompson, 32, was shot dead after wounding the two officers, one critically, on a busy commercial district in Queens. Although a police source had earlier cited the assailant as "just an angry guy," officials are now stating that Thompson had browsed al-Qaeda web sites and watched beheadings.

"This was a terrorist attack," Police Commissioner William Bratton said Friday, adding that even though Thompson had not been on a watch list, they had discovered evidence of social media postings where he spoke of injustices in U.S. society and abroad, but did not indicate any affiliation with a terrorist group or having sought training.

Around 2 p.m. Thursday, four police officers were stopped and asked by a freelance photographer to pose for a picture at the intersection of Jamaica Avenue and 162nd Street in the city's borough of Queens, a New York Police Department spokesman said.

A man can be seen walking on the sidewalk towards the cops with an 18-1/2-inch hatchet in his hand, according to a surveillance video. Suddenly he attacks the officers without any provocation, swinging the hatchet to strike one officer in the right arm and then swinging it the other way to strike a second officer in the head, the Associated Press reported.

Taking charge of the situation, the remaining two officers fired their weapons at the attacker and fatally shot him, but also ended up accidentally shooting a stray bullet at a 29-year-old female bystander in her lower back, police said.

Inspired by terrorist groups such as the Islamic State and al-Qaeda, Thompson allegedly became "self-radicalized, the New York Times reported.

"We at this time believe that he acted alone," Bratton said at a news conference, which was also attended by New York City Mayor Bill de Blasio, according to the Times. "We would describe him as self-radicalized. It would appear at this time that he was self-directed in his activities."

John Miller, who oversees intelligence and counterterrorism for the NYPD, said recent online history showed Thompson had visited websites related to those terror groups, the Times reported.

"It appears, just from the electronic forensic piece of this, that this is something he has been thinking about for some time and thinking about with more intensity in recent days," Miller said.

Additionally, Thompson's father informed authorities that his son had "spent extensive amounts of time by himself in his bedroom and by all accounts, was a true proverbial loner."

While 25-year-old officer Kenneth Healey was listed in stable but critical condition with a head wound at Jamaica Hospital on Friday morning, 24-year-old officer Joseph Meeker was listed in stable condition with an arm injury. Additionally, the accidentally wounded 29-year-old woman was recovering after undergoing surgery, police said.

The 32-year-old Thompson once served in the U.S. Navy, and had a series of run-ins with the law over domestic violence in California in 2003 and 2004, police said.