By Alex Dobuzinskis

LOS ANGELES (Reuters) - The Federal Bureau of Investigation on Wednesday offered a $50,000 reward for help in apprehending a serial killer suspected of 12 slayings and 45 rapes in California, in a 40-year-old cold case.

The unidentified man, who would now be between ages 60 and 75 years old, carried out his attacks between 1976 and 1986 throughout California and is considered one of the state's most prolific serial killers, according to the FBI. Officials described him as a white male, about 5 feet, 10 inches tall with blond or light brown hair.

Investigators have dubbed him the East Area Rapist and the Golden State Killer.

He was adept at using firearms and was known to sneak into a house and confront sleeping couples in bed, shining a flashlight in their eyes before raping the woman while the man was tied up, according to the FBI.

On occasion, he would kill the couple. Other times, he attacked women alone at home or with their children.

"There is a likelihood that he is alive and certainly if he is we would like to get some justice for the victims," FBI spokeswoman Gina Swankie said in a phone interview.

The first crime investigators attributed to the man was a rape in the Sacramento area on June 18, 1976, nearly 40 years ago.

"It's an ideal time to refresh everyone's memories and try to get this reward to encourage people to come forward with information," Swankie said.

Between 1976 and 1978, he carried out burglaries, rapes and killings in the Sacramento area, then centered his attacks on the eastern region of the San Francisco Bay Area in 1979, the FBI said in a statement announcing the reward.

Over the next two years, he killed several people along the California Coast, the FBI said.

After a 1981 slaying in the Southern California community of Irvine he appears to have stopped until 1986, when he struck again in Irvine, raping and killing a teenager, the FBI statement said. That was the last attack.

Investigators have linked him to 12 slayings and 45 rapes based on DNA evidence and his chillingly unique methods, Swankie said.

The killer also burglarized homes and took valuables from victims while they were bound, accumulating a trove of coins and jewelry, which led the FBI in their announcement to ask people to alert it if they discovered a "hidden collection of items" belonging to a friend or relative.

(Reporting by Alex Dobuzinskis in Los Angeles; Editing by Steve Orlofsky)