LOS ANGELES » A Los Angeles man has unexpectedly pleaded guilty in the samurai sword stabbing and bludgeoning deaths of a woman, her two children, her boyfriend and a NASA scientist, prosecutors said Friday.

Steve Kwon, 44, pleaded guilty Thursday as a jury was being chosen for his trial in the grisly 2008 killings in Quartz Hill, 40 miles north of Los Angeles. Prosecutors say Kwon and his best friend, Jae Shim, beat and stabbed Shim’s ex-wife, 34-year-old Young Park, her two children and the two men before setting some of the bodies alight and burning down the house.

Kwon and Shim had planned on framing Park’s boyfriend, Si Young Yoon, a 34-year-old tae kwon do instructor, but the pair accidentally left the baseball bat and samurai sword they used in the killings in the home before they set it on fire, said Beth Silverman, a prosecutor with the Los Angeles County District Attorney’s Office.

Kwon and Shim ditched their original plan to bury Yoon in the Angeles National Forest and fled to Mexico, where they dumped the body before their car got stuck in the desert, which eventually led to their capture and extradition to Los Angeles.

Silverman said the crime was one of the worst she’s ever seen.

"It’s horrific," she said. "Five people were bludgeoned and stabbed to death, and the mother and her two children were piled onto a bed and set on fire to the point they were unrecognizable and had to be identified through dental records."

Kwon’s attorney, David Sakata, declined to comment Friday.

In February, Shim pleaded guilty to killing Park and the others and agreed to testify against Kwon in a bid to spare himself from the death penalty.

Silverman said Shim and Park were in the middle of a custody battle over their son when he and Kwon killed her and her other two other children, who were 11 and 13.

Shim and Kwon also killed Joseph Ciganek, 60, who worked at NASA’s Dryden Flight Research Center at Edwards Air Force Base and was married to Park’s cousin, who was at work at the time of the killings. Park had moved into Ciganek’s home after she divorced Shim, and Silverman said the cousins were more like sisters, Silverman said.

Both Kwon and Shim face life in prison at their sentencing Aug. 21, when Park’s cousin and other family members of all the victims will get a chance to speak about how the murders have affected them.

"The two of them will die in prison," Silverman promised. "They don’t deserve to ever get out."