Frustrated at getting no solid leads in the most shocking homeless killing in recent years, San Francisco police have increased the reward to $30,000 for information leading to the arrest of whoever shot two people in a wooden box while horrified onlookers watched helplessly.

Lindsay McCollum and Eddie “Tennessee” Tate were living in their handmade, wheeled shanty at 16th and Shotwell streets when a pair of gunmen walked up on Dec. 18, 2016, at 8:46 p.m., opened fire and ran off, witnesses told The Chronicle.

Tate, 51, died in the box. McCollum, 27, stumbled into the middle of the street where she died, bleeding and convulsing, in front of stunned passersby and other homeless people camped at the corner.

McCollum’s parents, Mike and Carrie McCollum of Tennessee, last year offered $5,000 of their own money for tips in the case and put up posters throughout the slaying area with their email information. Most of what they got were “crazies” who were incoherent or nasty, Carrie said. The picture of Lindsay, a well-dressed woman with pistols tattooed on her chest, was cut out of the largest poster.

In late October, after a fresh round of inquiries from the McCollums, the Police Department offered $25,000 for leads, in addition to the $5,000 from the McCollums.

One difficulty is that the homeless people who knew Tate and McCollum have been tough to get hold of, said Officer Robert Rueca, a police spokesman.

“It’s a transient population and someone might have seen something and moved on,” Rueca said. “We hope this reward might jar someone’s memory.”

Tate, a roaming handyman, and McCollum, who had studied to be a veterinarian, were not a couple. They were living in the box purely for shelter, friends and family said. Both struggled with heroin addiction, which in McCollum’s case began after she was prescribed OxyContin for car-crash injuries.

“Lindsay’s death is such a terrible loss, but I also feel bad for all the homeless people who knew them,” Mike McCollum said. “Nobody in second grade raised their hand and said, ‘I want to be a homeless druggie.’ I just hope someone remembers something that can help.”

Tipsters are asked to call the homicide detail at 415-553-7976.

Kevin Fagan is a San Francisco Chronicle staff writer. Email: kfagan@sfchronicle.com Twitter: @KevinChron