Robert Charles Browne told investigators he used ether to knock out one woman he was trying to seduce, then "used an ice pick on her." He said he used ant killer to subdue another woman and stabbed her nearly 30 times with a screwdriver.

In all, the man already serving a life sentence for murdering a child told investigators he killed 49 people starting in 1970.

"None ever got away; never gave the opportunity," Browne, 53, told investigators during a series of prison interviews. "If you're going to do it, just do it."

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A 44-page affidavit released Thursday paints a picture of a killer who had a low opinion of women and thought he was justified in killing them, who met his victims in everyday, even mundane, situations — a motel bar, an apartment complex, even a convenience store where he worked.

Investigators so far have been able to corroborate Browne's detailed claims in six slayings — three in Louisiana, two in Texas and one in Arkansas, El Paso County Sheriff Terry Maketa said. Browne also pleaded guilty Thursday in Colorado Springs to another killing, that of a 15-year-old girl in Colorado Springs shortly before his arrest.

"It's possible he's exaggerating," Maketa said, "but I don't think you can conduct business assuming he's exaggerating."

Browne's public defender, Bill Schoewe, did not return calls seeking comment.

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.

Authorities said Browne grew up the youngest of nine children in the northern Louisiana town of Coushatta, about 40 miles southeast of Shreveport.

Browne, a high school dropout who got kicked out of the Army for drug use, said his killing spree began during a bar fight with a soldier in South Korea in 1970. Maketa said that claim 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, Maketa said.

Browne's father, Ronald, served as a parish deputy at the same time the department was investigating the death of 20-year-old Wanda Hudson, 20, in Coushatta, said Johnny Norman, the sheriff of Red River Parish in northern Louisiana about 40 miles southeast of Shreveport.

According to the affidavit, Browne confessed to stabbing Hudson nearly 30 times with a screwdriver in her apartment — a home he was familiar with because he had changed locks there as a maintenance man.

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. 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.

Investigator Charlie Hess said he believes the killer himself doesn't even 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," Hess said.

It was Browne who spurred investigators to take another look at his past when he sent an unsolicited letter to prosecutors in 2000.

"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."

The letter included a map traced from an atlas, and it showed outlines of Colorado, Washington, California, New Mexico, Texas, Oklahoma, Louisiana, Arkansas and Mississippi. Browne wrote a number inside each state and the total was 48.

Authorities responded, but Browne clammed up for a while, then agreed to more discussions. Eventually, he began providing details on other slayings.

Authorities say Browne never gave much thought to the killings, before or after. He forgot names and had trouble with details, once mixing up two victims during interviews with investigators.

He did have a "very low" opinion of women, investigators said in the affidavit.

"He said, 'women are unfaithful, they screw around a lot, they cheat, and they are not of the highest moral value. They cheat and they are users,'" they said. "Mr. Browne said in some way, he feels justified in what he has done."