Mourners returned on Monday to the scene of a shooting rampage at a bar in the Los Angeles suburb of Thousand Oaks, where a Chicago-area carpenter erected wooden crosses dedicated to each of the 12 victims.

Greg Zanis, who has been building crosses to remember shooting victims for more than two decades, said he arrived in Thousand Oaks, 40 miles northwest of Los Angeles, early on Sunday morning to meet the families and put up the crosses.

'I'm working for the families. I always bring a double set. I sand them, I paint them, I put hearts on them and I put their names on them. I also put their photo on there in black and white as a ghost. It's the most personal gift a carpenter can give to a grieving family,' Zanis, 67, told Reuters.

A former US Marine combat veteran opened fire last week in the Borderline Bar and Grill bar, which was packed with line-dancing college students, killing 12 people in a mass shooting that stunned Thousand Oaks, a bucolic Southern California community with a reputation for safety.

The gunman, identified by police as 28-year-old Ian David Long, was found dead of an apparently self-inflicted gunshot wound following the massacre.

Zanis said he had returned from Pittsburgh, where he had delivered 11 Stars of David in remembrance of the worshippers shot dead on October 27 at the Tree of Life synagogue, before setting out for Thousand Oaks in his truck filled with crosses.

The master carpenter said he began his one-man mission in 1996 when he found the body of his father-in-law, who had been shot to death.

In 1999, he erected 13 crosses in Colorado in honour of the victims of the shooting rampage at Columbine High School, where 12 students and a teacher were killed.

Zanis makes the crosses by hand and drives to each site in his truck, making a 4,000-mile round trip for the Thousand Oaks shooting. He accepts only small donations to help pay for gas.

He said he would stop in Las Vegas on the way home to deliver a cross to the brother of shooting victim Justin Meeks before starting on a series of crosses to memorialise victims of the California wildfires.

'I'm going to go home and start making as many as I can. I'm going to need a trailer to carry that many on my next trip,' he said.