Two months ago, police officers who patrol downtown Los Angeles took to the streets to film a visual get-well card for a fellow officer who had been injured dismantling a roadside bomb while on duty in Afghanistan with the Marines.

Buoyed by his friends' messages and by an accompanying music video dedicated to him, Staff Sgt. Joshua J. Cullins bounced back quickly from a concussion suffered in the July 16 explosion of the 15-pound bomb.

On Tuesday, his friends learned that Cullins, 28, had died the day before from injuries caused by another roadside bomb.

Details of his death were not immediately available from the Department of Defense, but police sources indicated that Cullins was killed by a secondary bomb as his unit was investigating an earlier explosion.

A Marine Corps reservist, Cullins had been serving with the 3rd Battalion, 6th Marine Regiment in Marja, in Afghanistan's dangerous Helmand province.

August's two-part "Welcome Home" video for Cullins chalked up more than 8,500 views on YouTube.com. In their messages, police officers ranging from Chief Charlie Beck to rookie patrolmen were seen telling Cullins that they were longing for the day they could welcome him home.

"There are no words to express how we feel," Capt. Daryl Russell said Tuesday. He commands patrol officers at the downtown Central Station and was Cullins' LAPD boss.

"I'm so sad this has happened to a kid who really had a bright future with this Police Department," Russell said. "This is a total loss to this city and this country."

Russell said Cullins was being courted by the department's bomb squad because of his expertise as an explosive ordinance officer with the Marine Corps Reserve.

He said Cullins returned to the field in Afghanistan as quickly as doctors would allow. "He'd put in his time and could have had a desk job," Russell said. "He continued wanting to be out with his fellow Marines, doing the job every day he strapped on his boots."

Russell commissioned the "Welcome Home" video. Officer David Marroquin, who has video production experience, recruited a friend, actor-musician J. Hunter Ackerman, to write and perform music for what became a four-minute, 40-second music video.

Footage was shot at the Salton Sea, Fairfax's Original Farmers Market and atop downtown's 52-story Gas Co. tower as well as at local police stations.

Marroquin hurried to Cullins' parents' Simi Valley home Tuesday after learning of the Marine's death, announced "with deep sadness" by Beck.

"It's hard to believe. Unfortunately, we will be welcoming him home in a different way than we expected and were hoping to," Marroquin said.

Cullins spoke frequently and exchanged e-mails with Marroquin after being surprised and delighted by the video and by his police colleagues' well wishes. The Marine posted his last entry on Marroquin's Facebook page Saturday.

"He was a guy who had a lot on his mind and he was asking about how others here are doing and telling us to be safe on the streets back here," Marroquin said. "He said he felt he wasn't worthy of the video and he hoped he wouldn't let everybody down by not making it home."

Cullins is the second LAPD officer to die in Afghanistan. Robert J. Cottle, a SWAT officer and sergeant major with a Marine Corps Reserve battalion, was killed March 24.

Cullins is survived by his parents, Jim and Barbara Cullins; and brothers, Cooper, 12, and Donovan, 16.

In August, Jim Cullins, who owns and operates Abe's Deli in Northridge, told The Times that his son joined the Marines a week after graduating from high school. He spent two years on duty at an embassy in Africa and later became interested in explosive ordinance disposal.

His son was deployed to Afghanistan, then Iraq before leaving active duty, Jim Cullins said. Joshua Cullins became a Marine reservist specializing in bomb disposal and joined the Police Department in July 2008.

Cullins said he worried about his son's Afghanistan work. "Yes, it does bother me a lot that my son takes apart bombs. But I've got one job only - to be there and let him talk to me. There's no sense in me getting emotional when he says he has given me his power of attorney," he said.

"He told me: 'Dad, remember this - if I get hurt it's not your fault. I got into this by myself.' "