Nicholas Glenn strode up Sansom Street just before midnight with a 9mm Ruger in his hand.
In his back pocket was an envelope with a feverish, rambling letter inside, and two words scrawled on front.
It was Friday night, and down the street at the Maximum Level Lounge, the door was open and the drinks were flowing. A few blocks away, a young woman and her boyfriend pulled up to her apartment. Philadelphia Police Sgt. Sylvia Young, a 19-year veteran of the force, parked her squad car at the corner of 52nd and Sansom. Her shift was almost over.
Glenn, a man with a long criminal record and an apparent grudge against law enforcement officers, walked up to Young's squad car and fired 18 rounds into the window.
Up and down Sansom Street, neighbors heard a burst of gunshots - then more and more still, as the 25-year-old, running from pursuing officers, sprayed four blocks with 51 bullets.
Glenn opened fire on the bar and the car where the young couple sat, police said. He killed Sara Salih, a 25-year-old community college student, and injured four more people, including a University of Pennsylvania police officer, before police officers shot and killed him in an alley off 48th Street.
As dozens of officers swarmed the scene, crime scene investigators pulled the envelope from Glenn's pocket.
Shaken city officials, addressing news crews on Saturday, said the carnage could have been much worse. Glenn had carried three clips of ammunition and a bag of about 15 loose rounds.
The incident marked the second time this year a Philadelphia police officer has been shot in an ambush-style attack, and came after targeted shootings that killed officers in Dallas and Baton Rouge this summer.
"A horrific night," said Commissioner Richard Ross on Saturday afternoon. "We know he was hell-bent on hurting a lot of people."
Young, 46, was hit six times, with gunshot wounds to the left shoulder, arm, and chest. The 22nd District veteran, who normally works in North Philadelphia, was on assignment with an anticrime task force in West Philadelphia on Friday night.
When Glenn approached the car, she leaned right, trying to dodge the bullets, police said. Some smashed into her body armor. Two hit her service revolver, rendering it useless.
"I'm at the McDonald's off 51st Street!" she yelled into her radio. "Help me, please!"
The first officers on the scene, also 22nd District patrol officers, found Young slumped in the front seat, pulled her into an arriving patrol car, and took off after Glenn.
Glenn ran down the street, past the Maximum Level, where, in the doorway, a security guard and a bartender were helping a patron in a wheelchair, police said.
He spun around. A law enforcement source said he grabbed the bartender and pulled her in front of him as a human shield.
"Don't chase me!" he yelled to the pursuing officers, the source said. Then he fired a shot into the woman's leg. The security guard, who the source said emerged from the bar with his own gun drawn, was shot in both legs. A police officer used a belt as a tourniquet to stanch the bleeding.
Glenn kept running. He passed a car dealership on the 4800 block of Sansom, ducked briefly inside - the night cleaning crew spotted him among the tires - and then ducked back out.
Across the street, a couple - Salih and her 36-year-old boyfriend - had just parked their car, a white Nissan Altima, outside Salih's apartment.
Glenn fired 14 bullets into the car, striking Salih and her boyfriend multiple times. Glenn kept running.
Dozens of squad cars sped to the scene. Police said Glenn fired at one coming down Sansom Street, hitting the hood and driver's side door but missing the officers inside.
Police radio buzzed with voices.
"We got two people shot out here!"
"We've got somebody shot in the car!"
"It's coming from everywhere down here."
And then: "We've got another officer down."
Glenn had darted into a weedy, L-shaped alley on Sansom Street. The 22nd District officer who had chased Glenn from 52nd Street followed, flushing him toward the end of the alley on 48th Street, where another group of officers were pulling up.
Janice Smith, in her kitchen on 48th Street, heard the helicopters whirring overhead, then a barrage of shots from the alley.
The living room "lit up like firecrackers" from the lights of the police cars outside, she said. Then an officer shouted: "Drop your weapon!"
Glenn had reloaded, police said, and fired at the officers in the mouth of the alley, hitting Eddie Miller, a University of Pennsylvania police officer.
The officers fired back. Glenn collapsed in the alley, shell casings surrounding him, a law enforcement source said.
The entire episode had lasted less than half an hour.
"Chaos," Smith said. "Pure chaos."
Miller, 56, the Penn officer, was hit in the right hip and ankle in the gun battle. He was cracking jokes at Penn Presbyterian Medical Center that night, police said.
Young was listed in stable condition at the same hospital.
The security guard and bartender at the Maximum Level Lounge were both listed in stable condition. The tourniquet likely saved his life, a law enforcement source said.
Salih was pronounced dead about two hours after she was found shot in her car. The exuberant, outgoing daughter of Ethiopian immigrants, she arrived in the U.S. as an infant and had lived on the block with her family for the last 10 years. She was studying to become a paralegal.
Her boyfriend, who was hit in the arm and the chest, is in critical condition but expected to survive.
Police were still investigating Glenn's background on Saturday.
But Glenn's "DOOMED PEOPLE" envelope and the letter inside held as many questions as they did answers, law enforcement sources said. Police said they believed that, like Edward Archer, the West Philadelphia man charged with a similar ambush shooting of Officer Jesse Hartnett in January, Glenn had acted alone.
The rambling, self-referential letter mentioned Glenn's probation officer by name - the only person specifically named - and expressed anger that he had not been assigned a female probation officer. He also expressed hatred for the police, officials said.
Glenn had a long criminal history of mostly drug-related offenses. According to court records, he had drug-possession convictions in Philadelphia. He had also been arrested and charged in connection with a gang rape in a November 2009 incident, but these charges were later dropped. He spent seven months in prison on a parole violation last year.
In his letter, he wrote that the world was doomed, that people were doomed, and that he was doomed, a law enforcement source said. It appeared he knew that Friday night would be his last, the source said.
"It was more about him than who he wanted to hurt," Commissioner Ross said in a news conference Saturday. Other than "the rantings in the letter," he said, Glenn's motive was unclear.
"We may never know," Ross said.
Staff writers Julie Shaw, Chris Palmer, Chris Mondics, Robert Moran, and Sam Wood contributed to this article.
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