Disturbing new video shows a deadly attack on a homeless man in San Francisco last month that police call one of the most horrifying they've ever seen.

Police are asking the public for help identifying the three young men seen in the clip minutes before they kicked and beat to death Tai Lam, a 67-year-old homeless man.

Lam was a 100-pound man who needed crutches to walk, and he appeared to have been sleeping when the trio first approached him, investigators said. There is no indication he provoked the three suspects, they said.

Veteran officers who said they’ve "seen it all" said this attack was bad.

"I have 25 years in, and this is a really awful attack, probably one of the worst I've ever seen,” San Francisco Police Sgt. John Cagney said. “I think all of us that are investigating this were personally affected by it."

The video surveillance footage recorded the suspects attacking Lam twice on Nov. 23, between 11 p.m. and midnight, in the 100 block of Sutter Street, according to San Francisco police Lt. Toney Chaplin.

"The fact that they left and came back and resumed the attack, I think it speaks volumes about how bad it actually was," Chaplin said.

Police said they think the men traveled on BART or Muni the night of the assault and predict they will find even more surveillance video of the trio.

Lam's body wasn't found until about 7 a.m. Nov. 24, according to police. The victim's family has been notified and asked for privacy, Cagney said.

Homeless advocates said it's common for the homeless to experience violence.

"The truth is homelessness is just a very, very dangerous way to live on so many different levels," said Claire Wagner of Home First. "I think this is one of the reasons why people gather in encampments. Some of them feel there really is safety in numbers."

Wagner also points out many people without housing are elderly and disabled like Lam.

Anyone with information can contact Cagney at 415-734-3831 or may do so via the anonymous tip line at 415-575-4444 or send a tip by text message to TIP411 with "SFPD" in the message.

Bay City News contributed to this report.