Olivia Dorsey was in her bedroom early Wednesday when commotion from the sidewalk startled her - voices that sounded like a man and woman arguing, she recalled, then an instant of silence, then the gunshot, "loud as a cannon."

Dorsey, 80, made her way outside to discover her gracious neighbor, Jose Ortiz-Diaz, shot down by a robber and dying in the arms of a friend.

"Jose was lying flat on his back, and his head was in this woman's lap, and she was rocking him like she was trying to wake him up," Dorsey said Friday, looking over her shrubs toward where Ortiz-Diaz fell. "She was screaming and screaming, 'Help me! Somebody help me! Oh, God, help me!' "

Ortiz-Diaz, 33, had been standing outside his rooming house on the 3200 block of North 15th Street in Tioga around 2 a.m., talking with a friend who was locked out of her home, police said.

The gunman pedaled up on a silver BMX bike, pointing a chrome-colored gun and demanding the woman's purse.

Ortiz-Diaz, stocky and fit, a fitness enthusiast who worked as a personal trainer, stepped in front of the woman and tried to wave down a passing car.

The car didn't stop, and the gunman fired once, striking Ortiz-Diaz in the chest. The killer pedaled away - "calmly," according to one witness - north toward Westmoreland Street, police said.

Homicide investigators described the gunman as black, about 5-foot-5, thin, in his 20s and wearing a black hooded sweatshirt. Detectives are reviewing surveillance camera footage. As in all city homicides, there is a $20,000 reward.

"This was an innocent victim," said Sgt. Bob Wilkins, who is leading the investigation. "We need the public's help."

Originally from Puerto Rico, Ortiz-Diaz moved to Philadelphia in 2001, his family said. He had rented a second-floor efficiency in the 15th Street rooming house for 10 years.

"He was a beautiful neighbor - just plain nice," Dorsey said. He was always courteous and smiling, and in the hot weather, he drove her to her medical appointments.

"He'd say, 'Oh, Mommy, I'll take you,' " she said.

He worked out, sometimes twice a day, at Bally Total Fitness on Aramingo Avenue and had worked there for a time as a personal trainer and masseur. To help support his 3-year-old son, he took second and third jobs, his friends said, including as a pretzel twister at the Philly Soft Pretzel Factory around the corner from his apartment, on Broad Street across from the Temple University School of Dentistry.

"He was a stand-up guy," said store manager Merrick McDonald, holding a memorial flier with Ortiz-Diaz's picture on it. "He had a lot of machismo."

A couple of years ago, somebody tried to hold up the Dunkin' Donuts next door, McDonald said, and Ortiz-Diaz made the evening news by holding the door shut from the outside, trapping the robber inside until police came.

On his off days, he'd walk by and shout inside, with a laugh, "Four hundred pretzels! Five hundred pretzels!"

Last Saturday, he bought a coworker's daughter a book bag - having searched in vain for Justin Bieber school folders - so the little girl would be ready for the first day of school.

His son, Jose Jr., was the center of his life, his friends said.

The little boy doesn't know his father is dead, his mother, Jennifer Cruz, said by phone Friday, while Jose Jr. could be heard playing in the background.

"I don't know what to tell my son right now," she said.

Authorities asked that anyone with information call the Homicide Unit Task Force at 215-686-3263.

Contact Mike Newall at 215-854-2759 or mnewall@phillynews.com, or follow on Twitter @MikeNewall.