PORTLAND — The Portland man who was killed Monday near the corner of Congress Street and Massachusetts Avenue died from a gunshot wound to the chest, the state medical examiner ruled after an autopsy Tuesday.

Allen MacLean, 41, whom family members described as a warmhearted and loving person, was killed before dawn Monday.

Witnesses said they heard a loud pop at 4:20 a.m. and saw MacLean running across Congress Street from 4 Massachusetts Ave., wearing shorts and no shirt, yelling, “I’m dying. I’m dying.”

“He was just flailing his arms … He almost got hit by a car and then he collapsed on the sidewalk,” said Melissa Broome, who was waiting for her supervisor to open the nearby convenience store where she works.

The car passed, and Broome ran across the street and saw that MacLean was bleeding heavily.

“I didn’t know what to do,” she said. “I don’t have any medical training, so I just held his hand and told him the ambulance was coming.”

Jim Bellanceau said he was waiting at another convenience store on Congress Street when he heard a shot.

“I heard a ‘pop,'” he said, and saw a man running across Congress Street and screaming for help. “By the time I got there, he was just barely breathing. He had blood all over the top of his head … I thought they beat him up.”

Bellanceau said he saw the wound, a hole the size of his finger, just above MacLean’s heart.

He said his wife, who was with him, saw two men running from the apartment building, one on Massachusetts Avenue and the other on Congress Street.

Police officers tried to resuscitate MacLean but could not. When paramedics arrived, they pronounced him dead.

Police believe MacLean was shot in an alley behind the three-story apartment building at 4-6 Massachusetts Ave. and collapsed on the sidewalk in front of the Mobil gas station and convenience store at 1196 Congress St. A trail of bloodstains leads from behind the building to the road.

On Tuesday, police continued searching for the person who shot MacLean and trying to determine the circumstances that led to the shooting. They were working to find out why MacLean, who didn’t live in the apartment building, was there and whether he was visiting tenants.

Police confirmed that they found a gun and other evidence nearby, and continued to explore whether drug trafficking played a role in the crime.

Neighbors have reported seeing many people coming to the apartment building for short visits, and frequent visits by taxi cabs. They also have reported seeing syringes on the grass outside the building.

MacLean had several minor scrapes with the law over the years, including theft and drug possession in 2009 and obstructing justice in 2010.