"It was a bunch of rantings that was tantamount to rambling so it wasn't the most lucid letter," Commissioner Richard Ross told reporters. "It was more about himself than who he wanted to harm."
Nicholas Glenn went on a deadly tear late Friday in Philadelphia, ambushing a police sergeant and firing rounds into a tavern and a car before police shot him dead, Ross said.
A 25-year-old woman died after she was shot seven times in the torso, police said. Two police officers and three others -- two men and a woman -- also were wounded.
Detectives are investigating whether Glenn was linked to or inspired by outside groups or accomplices, but so far there is no evidence that he did not act alone, Ross said. Glenn's 9mm Ruger pistol had the serial number obliterated.
"Right now we have a lot of questions. This is completely a bizarre situation," Ross said.
Police found a note at the scene that said Glenn hated police and probation officers and it mentions one probabion officer in particular, Ross said.
Based on the letter, Ross said, he suspected Glenn may have suffered from a mental illness. He also noted Ross had an extensive criminal history.
"It is just one of those things that is surreal," he said. "Obviously he was hell-bent on hurting a lot of people. We still aren't sure why."
Ross could not immediately confirm that Glenn was a suspect in a 2009 West Philadelphia gang rape case.
Sergeant shot in protective vest
The shooting occurred around 11:20 p.m. Friday when Glenn approached a police car and fired through the window at Sgt. Sylvia Young, who was sitting in her cruiser at 52nd and Sansom Streets in west Philadelphia, police said.
No words were exchanged, according to Ross.
Young, a 19-year veteran of the force, was struck in her left arm and her protective vest. She is in stable condition at a hospital, police said. Ross said she is talking and will be OK.
Officers nearby heard the gunfire and responded, chasing down the man as he fled.
Glenn was "clearly trying to assassinate her," said Ross, noting Glenn fired at close range 18 times. Two of the bullets hit and disabled Young's pistol.
"It is absolutely miraculous that she is alive," Ross said.
Cornered in an alley
The gunman fired into a bar, which had its door open, wounding a security guard, police said. He then grabbed a woman and used her as a human shield, shooting her in the leg, authorities said.
He discharged a further 14 rounds into a car, striking a man and woman inside, Ross said. The woman died and the man is expected to survive, he said.
Ross found the attacks on the bar and civilian vehicle equally baffling, saying the letter contained no clue about why Glenn would have targeted random civilians.
Glenn ran into an alley before a University of Pennsylvania police officer and two others from the city police cornered him. Police said they shot and killed him, and he was declared dead at the scene.
The university police officer was shot in the pelvis and the right ankle, police said, and he is in stable condition at a hospital.
Four officers were believed to have discharged their weapons, police said.
CNN affiliate WPVI
reported that "officers will be riding two to a car until further notice."
"Gun violence across this nation is absolutely out of control," Ross said. "This should not happen in this country. That is the bottom line. ...This is a country where we have to do better with gun violence, plain and simple."
Another ambush in January
In January, a gunman in west Philadelphia shot and wounded an officer sitting in his police car
. That happened not far from where Friday night's rampage began.
The shooter in that incident allegedly told police after he ambushed Officer Jesse Hartnett that he had pledged allegiance to ISIS, police said.
But "we have no reason at this point to believe" Friday's shooting "is related to any beliefs of a religious nature or anything like that, just feelings about police departments and police officers in general and probation officers," said Ross.