Kevin Cullen’s head hit the brick wall so hard, one witness said, she “felt it in my house, on the second floor.”

By the time Philadelphia police responded to the call for a robbery in progress Sunday evening, Cullen was on the ground, unconscious and dying, his pockets turned inside out.

The motive in the beating death was robbery, police said in a news release. His assailants were teens.

The witness, who only gave New York TV station WABC her first name, Donna, said the boys “ran away laughing like it was funny. [They were] saying ‘We got him.’ ”

No one has been arrested in connection with Cullen’s killing, which has rattled the Mayfair neighborhood in northeast Philadelphia. Police said initial rumors that two teens had been taken into custody for the 57-year-old’s death were not true. The search continues.

Matt Giampa, who lives in the Mayfair community, said Cullen was a regular site “up and down this neighborhood.”

Cullen’s older brother, Tom, told WABC the attack was unprovoked.

“He was viciously assaulted by a pack of youths,” he said. “They beat him into unconsciousness, from which he never recovered.”

Homeless people like Kevin Cullen can be some of the nation’s most vulnerable crime targets.

The National Coalition for the Homeless documented 1,650 acts of violence against the homeless by what it described as “housed perpetrators” between 1999 and 2015, according to a report released last year.

“These crimes are believed to have been motivated by the perpetrators’ biases against people experiencing homelessness or by their ability to target homeless people with relative ease,” the report says. “The crimes include an array of atrocities such as murder, beatings, rapes, and even mutilations.”

According to the report 428 homeless people were killed in the attacks during that 16-year time period.

“Perpetrators of these attacks were generally males under age 30; most commonly they were teenage boys,” the report concluded.

Last July, San Diego police arrested a man who they said committed a string of deadly early-morning attacks on homeless men. Two of the victims were set on fire.

By the time Anthony Alexander Padgett was in handcuffs, police said two homeless men were dead and two others were left with serious injuries.

“It’s scary to think this could happen to me,” 35-year-old Chris Moton, who is homeless, told the San Diego Union-Tribune. He said he and other homeless people had begun sleeping in packs as police scoured the city for the killer.

In Las Vegas, police say someone used a hammer to bash in the heads of two homeless men — suspected “thrill kills” carried out solely for the rush of taking another person’s life, according to the Review-Journal.

According to The Washington Post’s Derek Hawkins, police used a decoy to catch a suspect.

“The decoy mannequin was staged in a manner which would have made it impossible for [the suspect Shane] Schindler to have determined the mannequin was not a human being before he struck,” police said in the arrest report.

Police had a camera pointed at the scene. It showed Schindler walk past the decoy three times. Then he pulled a four-pound engineer’s hammer from inside a pizza bag he was holding.

The Las Vegas Sun reported he “swung the hammer with both arms to “generate maximum force to his blow.”

In Philadelphia this week, Cullen’s family, suddenly thrust into the spotlight, was trying to stress to the community their dead loved one was more than just a man who lived on the streets.

He had problems and battled homelessness, but he wasn’t a violent man, or even a mean one, Cullen’s younger brother, Mark, told the Philadelphia Inquirer.

“He’d be somebody you’d see on the street all the time, and maybe you’d help him out, maybe you wouldn’t,” said his younger brother Mark, 54, who told the newspaper he slept on the bunk beneath Kevin growing up. “But he wasn’t aggressive or mean or any of those things.”

Now family members are planning a community memorial and making funeral arrangements as they come to grips with Cullen’s violent death.

“It’s going to haunt me for a long time,” Mark Cullen said.

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