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.

Heather Dawn Church, 13, who was murdered by Robert Charles Browne is shown in this photograph released by the El Paso County Sheriff's department July 27, 2006. Browne, a Colorado man currently serving a life sentence for the 1991 kidnapping and murder of Church, claims to have killed 48 people across the United States, authorities said on Thursday. [Reuters]

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 six slayings - three in Louisiana, two in Texas and one in Arkansas, El Paso County Sheriff Terry Maketa said.

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 confession came after several years of correspondence and discussion between the killer and cold-case volunteer investigators, authorities said. Browne himself sent the first letter in "cryptic and poetic prose" in March 2000 to El Paso County prosecutors, officials said.

"Seven sacred virgins, entombed side by side, those less worthy, are scattered wide," the letter says. "The score is you 1, the other team 48. If you were to drive to the end zone in a white Trans Am, the score could be 9 to 48. That would complete your home court sphere."

Authorities responded with letters, but Browne clammed up for a while, then opened a new dialogue with investigators. Investigator Charlie Hess said he believes the killer himself doesn't even know why 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 the youngest of nine children in the northern Louisiana town of Coushatta, officials said. He dropped out of high school and served in the Army from 1969 to 1976, when he was dishonorably discharged for drug use, Maketa said. He was married six times, and authorities said all his ex-wives are still alive.

Red River Parish Sheriff Johnny 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, Norman said.

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.