A "rambling" note expressing hatred for police was found after a man opened fire on a Philadelphia police officer then went on a shooting spree, injuring a second officer, killing a woman and wounding three other people before he was shot and killed by police in an alley, authorities said Saturday.

Police Commissioner Richard Ross identified the gunman in the Friday overnight rampage as 25-year-old Nicholas Glenn, and said he had a 9mm Ruger and at least three magazines as well as a plastic bag with 13 to 15 live rounds.

"Obviously, he was hell-bent on hurting a lot of people," the commissioner said at a news conference, adding that "we aren't absolutely clear as to why."

A "rambling" note expressing hatred for police was found after at the scene of the violent event. Ross said earlier that police believe the note was written by the gunman; he said it expressed hatred toward law enforcement and named a probation officer.

"This rambling suggests that he clearly was trying to target a police officer, as he did ... so it just kind of makes it very clear to us what he was out there to do," Ross said.

He said police believe the gunman acted alone in the violent events, which he described as "completely bizarre."

The wild chase and shootout through the streets of Philadelphia began about 11:20 p.m. Friday when Sgt. Sylvia Young, a 19-year police veteran, was ambushed while sitting in her patrol car in west Philadelphia; she was shot a number of times in the arm and protective vest, Ross said.

"She didn't hear him say a word, just walked up on her and started firing," Ross said. "She did hear about 15 shots or so, and that's consistent with the scene, where we believe she was struck at least eight times."

Officers hearing the shots pursued the gunman, who then fired into a nearby bar, hitting a security guard in the leg, then grabbed a woman and used her as a shield before shooting her in the leg, Ross said. Moments later, the suspect shot into in a car, hitting a man and a woman in the chest. The woman, who was hit seven times, was pronounced dead just before 2 a.m. Saturday, police said.

Ross said two police officers and University of Pennsylvania police officer Ed Miller chased the man into an alley, where the suspect was shot and killed. Miller was wounded.

Both Miller, 56, and Young, 46, were in stable condition Saturday at Penn Presbyterian Hospital, as were the three other civilians hit by gunfire. Police said Miller was shot in the pelvis and right ankle.

Ross referred to the Jan. 7 ambush shooting of Officer Jesse Hartnett, who was ambushed as he sat in his cruiser at an intersection by a man who investigators said told them he was "following Allah."

"(Young) had to do something very similar ... that Officer Hartnett did, and that is pretty much lean over in the passenger seat to try to shield herself from as many as those rounds as possible," Ross said.

Aside from the officers, the identities of the other people injured in the spree were not immediately released. The suspect remained unidentified.

Mayor Jim Kenney praised officers and urged them to follow Young's example and wear their protective vests.