A man serving a life sentence in Colorado for murdering a teenage girl has claimed responsibility for as many as 48 slayings across the country dating back more than three decades, authorities said Thursday.

Robert Charles Browne, 53, told authorities the slayings occurred from 1970 until his arrest in 1995. He was in court Thursday to plead guilty to one of those killings — the death of another girl in Colorado in 1987.

Authorities so far have been able to corroborate his detailed claims in seven slayings — three in Louisiana, two in Texas, one in Arkansas and the two in Colorado, El Paso County Sheriff Terry Maketa said.

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He said Browne's claim of 48 murders could be credible.

"It's possible he's exaggerating, but I don't think you can conduct business assuming he's exaggerating," Maketa said. "We'll continue to pursue leads."

If Browne's claims prove true, he would be one of the most prolific killers in U.S. history.

Gary Ridgway, Seattle's Green River Killer who in 2003 became the nation's deadliest convicted serial killer, admitted to 48 murders but once said he killed as many as 71 women, according to interview transcripts.

Browne's public defender, Bill Schoewe, did not return a call.

Browne claims his killing spree began with a soldier in South Korea in 1970, which Maketa said has not been verified.

The other claims include 17 murders in Louisiana, nine in Colorado, seven in Texas, five in Arkansas, three in Mississippi, two each in California, New Mexico and Oklahoma, and one in Washington state — 49 in all, the sheriff said.

Browne pleaded guilty in 1995 to kidnapping and murder in the 1991 death of Heather Dawn Church, 13, of Black Forest, a town north of Colorado Springs. He was sentenced to life without parole. On Thursday, he pleaded guilty to first-degree murder in the death of Rocio Sperry, a girl who was about 15 at the time of her death 19 years ago.

The sheriff's department said the charge in Sperry's death followed years of correspondence and discussion between the killer and cold-case volunteer investigators, including Lou Smit and Charlie Hess.

Hess said he believes Browne himself doesn't know why he is confessing.

"Does he have a conscience? Is that what motivated him? I really have no idea and I'm not sure he knows," said Hess, a retired police officer who also worked with the CIA and FBI.

Browne grew up as one of 12 children in a hardscrabble family in the northern Louisiana town of Coushatta in Red River Parish, said Johnny Norman, sheriff there.

Norman, a schoolteacher in the 1960s, said Browne was in his physical education class in eighth and ninth grades. He recalled Browne as smart but aloof — and with a short fuse.

"He was a loner, but not somebody you'd expect to do this. But he did have a hot temper," Norman said. "In a pickup basketball game, somebody fouled him or hit him, he'd fly off the handle."

The Browne family ran a dairy in the 1960s and had hard times.

Browne's father, Ronald, was a deputy sheriff at the time the department was investigating the death of Wanda Hudson, a woman in her 20s, the sheriff said. Browne has confessed to that slaying, authorities told The Gazette newspaper of Colorado Springs, which first reported Browne's claims.

Norman said he has spoken with Colorado investigators and the Louisiana State Police about Hudson's death and Browne's possible involvement.

"We never close a case," he said.