They weren’t there to rob Tai Lam.

The 67-year-old disabled homeless man — who, at 5′ 5″, weighed less than 100 pounds — was tucked into an alcove on Sutter Street in San Francisco, slumbering in his sleeping bag when three men approached him at around 11 p.m.

According to his family, Lam didn’t have psychological problems, nor was he addicted to any substances that might alter his behavior. As the San Francisco Chronicle noted, he was conscientious, choosing to urinate into plastic bottles to keep the alley where he slept clean.

But these three men decided to beat him mercilessly anyway, seemingly simply for some twisted version of fun.

Surveillance video caught the three men stomping on Lam. At one point, they walked away only to return and continue their savagery.

The video showed one of the assailants brutally kicking Lam, who cowered defenselessly on the ground. The assailant casually paused when a car drove by, as if he’s stopping a pickup basketball game for traffic. As soon as it passed, he resumed the beating.

Eventually the three sauntered out into the night.

And no, they weren’t there to rob him. As police pointed out, Lam had more than $1,000 on him.

The men didn’t even bother to take it.

Lam lay in the alcove, critically injured but alive. Police know because an autopsy revealed he asphyxiated on blood at some point. It’s unknown how long he suffered.

Police found his body the next morning.

That happened on Nov. 23, 2014. At the time, Sgt. John Cagney told the San Francisco Chronicle, “I’ve been a cop for 25 years, 10 in homicide. This was terrible. They were stomping on him. They were drop kicking the guy. And he absolutely did not do anything. He’s 100 pounds — a little, tiny, disabled Asian guy.”

According to Cagney, Lam was “beaten almost unrecognizable.”

Cagney added: “This is one of the worst attacks I’ve ever seen. Savage is the only way to describe it. All of us were really affected by this.”

Lt. Toney Chaplin agreed, telling the newspaper he hadn’t seen such a brutal slaying in his 25 years with the San Francisco Police Department.

“He was unable to defend himself,” Chaplin told KTVU. “He died alone after suffering these horrendous attacks.”

Both men desperately wanted to find the three attackers, going so far as to release videos of the suspects to the public in hopes that someone might recognize them. As KRON noted, one showed the three smoking cigarettes on the steps of Crocker Galleria just before the assault. The second showed the beating. While the videos raised outrage, they didn’t produce leads.

DNA found at the scene wasn’t a match for anyone in police databases either, meaning the men involved didn’t have any felony arrests in their past.

The case soon grew cold, and it seemed as if justice wouldn’t be served.

Cagney, though, wasn’t going to quit looking. In 2014, he said, “We’re going to get them.”

And he meant it. Two years later, his perseverance has paid off.

He refused to give interviews, but police gave the Chronicle the basic details.

Earlier this year in Washington state, a 21-year-old man named Joseph Stull was arrested for stealing a car. When he was being booked, a DNA sample was taken per standard operating procedure. It was entered into a law enforcement database, and, after two years, the San Francisco Police Department finally had a match for DNA taken from Lam’s slaying.

By the time Cagney was prepared to interrogate Stull, he had been arrested on further charges — this time in Indiana. He was in custody in Kootenai County on charges of grand theft and possession of stolen property, among other charges.

So Cagney flew to Indiana to question him. Stull offered details, telling Cagney that the attack wasn’t provoked but was completely arbitrary, the Chronicle reported.

Stull was arrested on Sept. 22, KTVU reported.

That interrogation later led Cagney to 21-year old David Peters, who was living in Stockton, Calif. On Oct. 25, he was also arrested.

Both men have been booked on charges of murder, robbery, elder abuse and assault with great bodily injury, KRON reported, and are being held on $5 million bail each.

The two have an arraignment hearing on Wednesday at 9 a.m., according to KTVU.

When the crime originally occurred, Cagney was flummoxed.

“I don’t want to say anybody deserves to get killed,” he told the Chronicle at the time. “But in 95 percent of the homicides, they’ve made some choices where there was a potential for bad things to happen. Even when it is random, there’s something you can point to. You cut somebody off in traffic, you have an argument. Somebody might hit you or kick you, but not a sustained beatdown. I just don’t get it.”

But mortality rates for the homeless in San Francisco are much higher than average.

Though it applies to young homeless, as Michael E. Miller reported in The Post, “’Homeless youth in San Francisco experience a mortality rate in excess of 10 times that of the state’s general youth population,’” according to a recently published study.”

Brutal slaying in Golden Gate Park stirs San Francisco’s raging debate over homelessness

The city’s mayor, Ed Lee, called homelessness a “visible and pressing concern” in May. So pressing that even Sen. Bernie Sanders (I-Vt.) commented on it that same month while he was campaigning for the Democratic presidential nomination. According to the San Francisco Examiner, he said, “I’ve just been in San Francisco for a few hours now but it really is stunning to see the number of people in this city who are sleeping out in the street.”

And younger homeless individuals aren’t the only ones at risk. Like Lam, many have found themselves the victims of violence while sleeping outdoors.

Jennifer Friedenbach, the executive director of the Coalition on Homelessness in San Francisco, has firsthand knowledge of just how dangerous.

“It is incredibly vulnerable to sleep outside,” Friedenbach told the Chronicle. “Homeless people say they have to sleep with one eye open. It’s really common for people to come into our office who have been severely beaten when they were sleeping.”

She claimed that she sees about one victim a week but that the numbers are much higher. Additionally, she said those most at risk are those like Lam — elderly and disabled.

“Most of the people who are without housing are disabled and elderly, like Tai,” she said. “All these are vulnerable populations that are unable to defend themselves.”

While two suspects have been arrested, the third from the video remains unidentified.

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