Man who killed wife, stepdaughter now charged in 1988 cold case murder, may be linked to 5 other slayings: Police Regis Brown is serving a life sentence for killing his wife and stepdaughter.

A Pennsylvania man behind bars for killing his wife and stepdaughter has now been charged in a 1988 cold case murder -- and may be linked to five other slayings, according to authorities.

Regis Brown, 59, is serving life without parole after he was arrested this March for the brutal slayings of his wife, Michelle Brown, and stepdaughter, Tammy Greenawalt, Pennsylvania State Police trooper Joe Vascetti told ABC News on Thursday.

Immediately after his arrest for the March crimes, Brown confessed to the cold case killing of 45-year-old Bryce Tompkins, Vascetti said.

"He has either confessed to, been arrested for, or is a strong suspect in eight homicides right now," including those three, Vascetti said. He declined to release more information citing the ongoing investigations.

Brown was charged with Tompkins' murder on Wednesday.

Tompkins was shot twice in the back on Dec. 26, 1988, according to the criminal complaint filed Wednesday.

Hunters found him dead, partially submerged in the Neshannock Creek in Lawrence County, police said.

Back in 1988, Brown was one of two suspects who was determined to be possibly involved, according to the criminal complaint.

Brown and the second suspect, Paul Ayersman, were arrested in Dec. 1988 for a series of robberies and burglaries, according to court documents.

Ayersman, who died two years ago, was interviewed in Feb. 1989 and indicated Brown admitted to killing Tompkins, according to the probable cause affidavit.

Ayersman was willing to cooperate at the time, but then he and Brown escaped from the Lawrence County Jail in March 1989, according to the probable cause affidavit. When they were arrested that July, the Lawrence County District Attorney at the time refused to give Ayersman any plea bargain because of the escape.

State police monitored Brown's actions over the last 10 years, the probable cause affidavit said. Brown was interviewed in 2014 about his involvement in Tompkins' killing and another murder that took place in 1986, but he denied involvement in both crimes, the probable cause affidavit said.

Once Brown was arrested for the killings of his wife and stepdaughter, in a March 19 interview with state police Brown admitted to killing Tompkins because Tompkins was a witness in a burglary and he thought he would tell police, according to court documents.

When Brown and his co-conspirator saw Tompkins a few days after the burglary, Brown shot him with a stolen gun and the two men dumped his body at the creek, the court documents said.

Four people interviewed by state police between 1991 and 2009 said Brown admitted his involvement in Tompkins' killing, according to the probable cause affidavit.

Brown, who is in a Pennsylvania prison, pleaded guilty in March to first-degree murder and third-degree murder in the deaths of his wife and stepdaughter. He was sentenced to life without parole, his attorney from that case, Eric Hackwelder, told ABC News.

Hackwelder has not spoken to his former client since Wednesday's charges, but he added, "I know he has been cooperating with investigators, from what I hear."

In connection with Tompkins' killing, Brown has been charged with criminal homicide, two counts of aggravated assault and intimidation of a witness, police said. He has yet to make a first appearance in court on these charges, said Vascetti.