WHEN Sgt. Chauncey Ellison came to Renovo Street, he was a father with a gun, trying to stand up to the young men he believed had assaulted and punched his 14-year-old son and robbed him of a pizza.

What happened on this dark one-way street in West Oak Lane that Nov. 17 night is subject to debate.

But this much is clear. Ellison, who was off duty, fired his police-issued Glock at Lawrence Allen, 20, who lives on the block, striking him once in the back.

Now Allen is fighting for his life at Albert Einstein Medical Center. The bullet punctured his lung and fractured his spine in nine places. He's paralyzed from the waist down, maybe the chest. He's battling pneumonia and struggling to breathe on his own, according to his family.

"My brother should have never been shot," his sister Lauren Allen said. "Never. It was for nothing - for a pizza he didn't take."

Ellison's son was robbed and punched in the face outside Bruno's Pizza at 19th and Cheltenham Avenue around 9 p.m., according to police, who said the boy had told his dad what happened.

Ellison confronted a 16-year-old known as "Meaty," and Meaty ran, according to Allen's family. Carrying a pizza, Meaty, a friend of Lawrence Allen's, sprinted up Renovo Street near 20th - less than two blocks from Bruno's - to the Allen family home about 9:05. Meaty told them that he had robbed a kid and that the dad was after him, the family said.

Ellison spotted some young people on Renovo Street and believed that they had been involved in the strong-arm robbery, said Lt. Frank Vanore, police spokesman.

Ellison attempted to stop one of the men, later identified as Allen, "and a struggle ensued," Vanore said. "He [Ellison] indicated that he believed [Allen] was drawing a firearm." Ellison, who joined the Police Department in August 2000, then drew his weapon and fired once, Vanore said. No gun was recovered. Friends and relatives took Allen to the hospital.

Witnesses, including two neighbors unrelated to the Allen family, said Ellison, wearing a red hooded sweatshirt, slowly drove away. Neighbors and relatives said they called 9-1-1. Ellison returned to the block after police arrived and spoke with officers, family and witnesses said.

Allen's relatives, three of whom were witnesses, say that Allen didn't commit the robbery and that there was no physical struggle whatsoever. They insist he had no weapon and did not appear to have reached for anything.

They say Ellison, behind the wheel of a black SUV with tinted windows, drove the wrong way up Renovo Street. A woman, identified by police as fellow Officer Robin Fortune, who was also off duty, was in the passenger seat.

Ellison and Fortune got out of the car. They seemed convinced that Allen had been involved in the robbery and Fortune began to scream at Allen and told Ellison to stand up to him, according to relatives and witnesses.

"You're going to let them do this to your son," she screamed, according to Lauren Allen, 21, who was standing near her brother at the time. "They're punk-a-- n-----," Fortune said, according to Allen.

Ellison, Lauren Allen said, remained silent.

"You ain't doing nothing," Fortune bellowed at Ellison, according to Allen. "You're just standing there."

"My brother started to turn around," Allen said. "He took his wife's hand and told her they should go in the house."

But Ellison, she said, "reached over me and fired his gun."

Joy Herder, 34, who lives on the block, said she had seen the entire incident from her second-floor-bedroom window.

She said she was on her computer when she heard an argument on the street below. She went to the window and saw a woman, later identified as Fortune, "jumping up and down," and screaming.

Both Fortune and Ellison stood near the SUV. Fortune's anger, she said, was directed at Lawrence Allen.

"She said, 'You don't know who you're f------ with,' " Herder said. "She was totally out of control."

Ellison, Herder said, had a gun in his hand. Allen had his hands at his side. "They were spread out and down," she said.

Ellison got closer to Allen, who is about 5 feet 5. "They were inches from each other. . . . The man with the gun was a lot taller than the victim," she said. "He [Ellison] had the gun up, pointed down to [Allen's] back," she said.

"He shook his gun and [Allen] was leaning against a car. He had nowhere to go," said Herder, who said she has given a statement to detectives.

Fortune, she said, kept screaming. "She was egging [Ellison] on," Herder said. "I think he just snapped."

"It was totally uncalled for," Herder said. Herder said she never saw Allen with a gun. "He never reached for anything," she said. "I'm 100 percent sure of it."

Herder heard one shot and watched Allen collapse first against the car, then to the street. "After he was shot, I left the window to call 9-1-1," she said. "When I came back to the window, the SUV was gone."

She watched family and friends help Allen into a car. Later when police came to the block, she learned that the man who had pulled the trigger was a cop.

"He should have known how to handle the situation," Herder said.

The Internal Affairs Bureau of the Police Department and the District Attorney's Office are investigating. Ellison, who works nights in the 22nd District, in North Philly, is on desk duty, standard procedure when a police officer is involved in a shooting. Fortune, who also works at the 22nd District and has been a Philadelphia officer since March 1997, has not been placed on desk duty. "She did not discharge a weapon," said Chief Inspector Anthony DiLacqua, who heads the Internal Affairs Bureau.

The D.A's office will determine whether Ellison should face any criminal charges in connection with the shooting or be cleared of wrongdoing.

Ellison said he had no comment early yesterday morning when asked about the case. Messages left for Fortune by the Daily News were not returned.

Allen faces charges of aggravated assault, simple assault, recklessly endangering another person, hindering apprehension and weapons offenses. Meaty, whose real name is being withheld because he is a juvenile, is charged with robbery, theft, terroristic threats, simple assault and recklessly endangering another person.

"Lawrence Allen is wrongfully charged and he's had his life destroyed," said Arnold Silverstein, who, with attorney Charles Auspitz, is representing Allen in the criminal case.

"If there was an alleged gun, why didn't Sergeant Ellison recover it?" Silverstein said.

"A bullet to the back is very troubling," he said. "This case makes me very angry. Everyone in the family is devastated by it."

Besides the current charges, Allen has no criminal record as an adult.

At age 17, Allen was arrested on assault and gun charges. He was found guilty as a juvenile for the illegal possession of a firearm and recklessly endangering another person. He was placed in a juvenile facility for 10 months, according to a law-enforcement source.

Two police officers keep watch in Allen's hospital room because he is under arrest, said Allen's mother, Terry Bowen, a mother of four who works as a nurse. He remains unconscious.

"If we say his name, he opens one of his eyes. That's all," Bowen said.

Allen has two children, a 2-year-old and a 4- month-old, with his wife, Rosemily "Rosie" Rosado, also 20. Rosado said she's pregnant with their third.

Rosado and Allen, who was unemployed, were home with their children the night of Nov. 17 when Allen's 16-year-old friend, "Meaty," came to the door.

"He told us he'd just hit someone, punched a boy in the face outside Bruno's Pizza," she said. "He was being a stupid kid."

Meaty was out of breath and had the pizza in his hand, said Lauren Allen, who was also home.

"He said the boy's father came up to the pizza shop and pointed a gun at him and said the guy was going to shoot him," Rosado said.

Meaty ran away. He told Lauren Allen and Rosado that the father was chasing him.

They went outside. Lauren Allen said she asked Meaty why he keeps doing "dumb stuff." "He started laughing and I hit the pizza and it went on the sidewalk under a car," she said.

Another sister of Allen's, Mecca Drake, and Meaty were the first to see a black SUV driving slowly up Renovo Street the wrong way, Lauren Allen said. "Get down, get down," Drake yelled.

The SUV stopped a couple of houses away from Allen's rowhouse in the middle of the street.

Ellison pointed the gun out the window, Rosado said, then both Ellison and Fortune stepped out of the SUV and Fortune started to yell.

They tried to reason with Ellison and Fortune, Rosado said. "[Allen] kept saying, 'It's over a pizza. I have nothing to do with this. . . . But if you want, I'll buy your son another pizza.' "

After Allen was shot, his wife became hysterical, Lauren Allen said.

"I kept screaming, 'Someone help me! Someone help me,' " Rosado said.

When they helped Allen into a neighbor's car, the back door wouldn't close. "We realized his leg was in the door, but he didn't feel it. He said he couldn't feel his legs," Lauren Allen said. They tore off his black shirt and wadded it up to try to stop the bleeding.

"The man who did this to my husband got in the car and left. He didn't say, 'Oh, my God, I'm sorry!' Nothing," Rosado cried.

"Not one word," she said, sitting on a couch, her voice quivering. "He shot him and left.

"How do you do that?" said Rosado, her body shaking, her head hung low resting in her hands.

"I want to know how." *