A man and his friend have been charged with stabbing and beating the man's ex-wife, her two children and her relative, a NASA scientist, before setting fire to a Mojave Desert home to cover up the crime.

Jae Hwan Shim, 39, and Steve Kwon, 37, were each charged with four counts of first-degree murder and one count of arson and burglary, the Los Angeles County district attorney's office said Tuesday.

The two were arrested Saturday in Douglas, Ariz., near the Mexican border, and were being held without bail.

Shim was accused of threatening his former wife, Young Park, after she left him earlier this year and moved in with her cousin in an unincorporated area near Lancaster, north of Los Angeles.

The burned bodies of Park, 34; her son Justin, 9; daughter Jamie, 13, and her cousin's husband Joseph Ciganek, a 60-year-old NASA scientist, were found in the house on June 23. Shim was not the father of the two children killed, but did have another child with Park.

A samurai sword and baseball bat were found in the home, and investigators believe they were used as murder weapons.

Meanwhile, the slain woman's boyfriend, Si Young Yoon, was missing and authorities were trying to determine whether he was a victim of foul play.

Yoon, 34, was a martial arts teacher for the children and prosecutors claim Shim was upset that his ex-wife was dating him.

Yoon's SUV, with his driver's license, check book and credit cards inside, was found near the burning home, authorities said. He originally was considered a "person of interest" in the killings but investigators changed their minds.

"They do believe he may be a victim of foul play now," sheriff's Deputy Denise Fuchs said.

Shim and Kwon would be eligible for the death penalty if convicted because the murder charges carry special circumstance allegations that they committed multiple murders and killed during a burglary.

Prosecutors will seek their extradition but the process could take several weeks, said Jane Robison, a spokeswoman for the district attorney's office.

Ciganek worked at NASA's Dryden Flight Research Center at Edwards Air Force Base, authorities said.

Sheriff's homicide detectives found Ciganek's body in one bedroom and the other victims in another. He and the children died of multiple stab wounds and blunt force trauma, the coroner's office said. Park died of multiple stab wounds.