A homeless man was found slain behind a busy fast-food restaurant in Anaheim late Friday, the fourth such killing in Orange County in the past month, and police blanketed the area in a search for an elusive serial killer.

“We have several detentions going on, and we have a search of the scene going on,” said Sgt. Bob Dunn, an Anaheim police spokesman.

Three other homeless men have been stabbed to death in recent weeks, and authorities have said the crimes are believed to be the work of a single serial killer.

Dunn said the lates t victim was believed to have been killed about 8 p.m. Friday. He declined to say what type of weapon was used, and said that while there were detentions, no one had been identified as a suspect.

The victim was not identified by authorities, but Krista Schegetz, 42, of Anaheim said she knew the man and said his first name was John. She said he was the only homeless man who hung out at the Carl’s Jr. restaurant.

Appearing shaken and teary-eyed, she said she had known him for six years and gave him care packages at Christmas and Thanksgiving. “He couldn’t be nicer,” she said.

Police cordoned off a large area around the crime scene, which was near the intersection of Imperial Highway and La Palma Avenue — a busy commercial area of restaurants, a movie theater and grocery store.

The search was spread over a mile-wide area, but appeared to be focused on a nearby mobile home park, which was swarming with police and dogs, according to reports from KCAL-TV news. The station said the body was found near dumpsters behind the Carl’s Jr.

The series of killings has prompted a major manhunt, with police from Anaheim, Placentia and Brea joining forces in the investigation and stepping up patrols in areas known for attracting the homeless. Detectives have enlisted the public’s help, releasing grainy footage from a security camera showing a man “lying in wait” at about the time of the first attack, in Placentia on Dec. 20.

That attack claimed the life of James Patrick McGillivray, 53. Investigators said they were looking for a white, four-door, late-model Toyota Corolla seen in the surveillance video.

So far, the primary lead is footage from a security camera captured at the time of the attack on McGillivray. It shows a man police described as dressed in black and “lying in wait.” Police said he appears to be about 18 to 25 years old, with a thin build and of average height.

On Dec. 28, Lloyd Middaugh, 42, was found near a riverbed trail in Anaheim. Two days later, Paulus Smit, 57, was killed outside the Yorba Linda city library.

Police have been urging homeless people to seek shelter and have gone into homeless encampments throughout Orange County warning those living there that a killer is on the loose.

Authorities have also conducted unusual roadblocks.

On Tuesday, police stopped and interviewed drivers across the street from the Placentia shopping center where McGillivray was killed, looking for any information in the case. They did a similar roadblock Friday afternoon outside the Yorba Linda library — hours before and a few scant miles from the scene of Friday night’s slaying.