The senseless, pitiless killing of 67-year-old Tai Lam, a homeless man who suffered from Polio and used crutches to move about, is difficult to contemplate. Lam was beaten to death by a group of three men in November 2014 in a Financial District alley near Montgomery and Sutter. Those suspects were captured on camera nearby, and in the weeks following the crime police revealed that Lam died with $1,000 on his person, leading investigators to hypothesize that robbery was not the motive behind the attack.

Nearly two years later, the Chronicle has news of a break in the case. Two 21-year-old suspects, Joseph Stull and David Peters, have been arrested, Stull in Idaho, Peters in Stockton. They were booked on charges of murder, robbery, elder abuse, and assault with great bodily injury. Each is held on $5 million bail.

“I’ve been a cop for 25 years, 10 in homicide,” SFPD Inspector John Cagney told the Chronicle. “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 (polio) Asian guy.”

The case was broken with DNA evidence. On charges regarding a stolen car, Stull was arrested in Washington state and a DNA sample was taken from him. The match came through, and later Stull was taken into custody in Idaho on car theft charges there. Cagney flew out to interrogate him, and Stull appears to have implicated himself in or confessed to the crime, providing explicit details of the beating.

Chillingly, Stull has not given what might constitute a motive, describing what occurred instead as a simple, random act of violence. Stull's account helped identify Peters, whom Police say also offered details of the assault and his involvement in it.

Related: Video: Homeless Man Beaten To Death In SF Alley Was Carrying $1000