SIOUX FALLS - Murder charges will be dropped against a prison inmate accused of killing two Vermillion girls in 1971 because a snitch made up the supposed admission, Attorney General Larry Long said Friday.

David Lykken, 53, pleaded not guilty to six alternate counts of murder for the May 29, 1971, disappearance of high school juniors Cheryl Miller and Pamella Jackson of Vermillion. The 17-year-olds were last seen driving a 1960 Studebaker on a rural Union County road on their way to a party.

Lykken's trial was to start March 25 in Elk Point. Instead, charges will be dismissed Monday and will not likely be refiled because the other evidence against him is not significant enough to proceed, Long said.

"I think if we had been able to do that, we would have left the charges in place. I don't think at this time we can recharge," he said.

Lykken is serving a 225-year prison sentence for the unrelated 1990 rape and kidnapping of a former girlfriend in Vermillion.

Another inmate, Aloysius Black Crow, told investigators he made secret recordings of Lykken in which he acknowledges killing Miller and Jackson, but it was really the voice of a third inmate who pretended to be Lykken, Long said.

"Black Crow manufactured a tape and told us it was Lykken and in fact it wasn't Lykken. That's the short and dirty version of it," he said.

The name of that third inmate was not released. But two anonymous sources told The Associated Press it was William Eutzy Sr., a 68-year-old inmate from Florida serving a life sentence in South Dakota to be closer to family.

Department of Corrections spokesman Michael Winder confirmed Eutzy has been incarcerated in South Dakota since Nov. 13, 1996.

He was convicted of first-degree murder in July 1983 for shooting to death a taxicab driver Feb. 26, 1983, in Pensacola, Fla., said Donna Oquist with the Escambia County, Fla., clerk of court's office.

Eutzy was sentenced to death but was re-sentenced in 1992 to life in prison without the possibility of parole for 25 years, she said.

Long said the third inmate denies being involved and he couldn't legally say if Black Crow had confessed, nor whether he would be charged with perjury.

Lykken's attorney, Mike Butler, and other members of the defense team said they knew immediately that the voice on the recording was not Lykken's because it had a higher pitch, lower cadence and was raspy from smoking.

It also should have been clear to detectives, he said.

"This case was a hoax from the beginning," Butler said.

Long, who was out of town for much of Friday, said he had not had time to talk to investigators about why they only recently suspected their witness was lying.

"I haven't gotten the chance to ask those questions of the right people yet. We're going to have to do a post-mortem on our side," he said.

"We get lied to all the time. We are the cops. And we're pretty good at spotting it. And why we missed this one we're going to try to find out."

Butler said he has no ill will toward investigators or prosecutors but believes the state owes Lykken and his family an apology for accusing him of the crime, alleging that his elderly mother and brother conspired in the case and for tearing up the family's yard in searches that yielded no clues. Officials also should apologize to the families of both girls because they were led to believe the case was nearing closure after 37 years, he said.

Long said he could not comment on that.

Besides not doing recognition tests to determine whether it was Lykken's voice on the recording, investigators ignored other evidence that could have called into question the case long ago, Butler said.

A December 1971 report provided to the defense indicated Jackson's parents and others positively identified her in a photo taken at a Sioux Falls concert six months after she disappeared, Butler said.

Detectives then concluded the girls ran away and stopped pursuing leads until the state's newly formed cold case unit looked into it in 2004, he said.

The investigation pointed to Lykken based on an interview from his sister, Nancy Bell, who was hypnotized and told about repressed memories she had of seeing the girls and their car on the family farm.

Black Crow was also an informant and key witness in another Union County cold case that resulted in the conviction of James Strahl of Dakota City, Neb., for the May 1998 beating death of William O'Hare at his Beresford farm house.

An Elk Point jury convicted Strahl of grand theft and first-degree murder, and he was sentenced in August to mandatory life in prison.

Black Crow testified that Strahl confessed to killing O'Hare after O'Hare refused to give him a ride back to Sioux City, Iowa, where the two men met at an adult bookstore.

Black Crow had been moved from Montana to South Dakota after snitching on an inmate and fearing for his safety. He acknowledged to snitching on inmates in other murder cases and did so in hopes of reducing his 90-year sentence and getting an early parole date.

Unlike with the Lykken case, investigators had physical evidence that pointed to Strahl, including his fingerprints on a can of beer and on a potato chip bag found in O'Hare's house and DNA on a cigarette butt in O'Hare's station wagon, which was found in South Sioux City, Neb., blocks from where Strahl was living.

Black Crow also told investigators that Strahl raped and killed runaway Amanda Gallion after she disappeared from Gillette, Wyo., in 1997 at age 14.

Strahl's lawyer, Phil Peterson, said since the Strahl conviction is on appeal, he'll ask the South Dakota Supreme Court to send it back to Union County.

"Once that's completed, we will file a motion for a new trial," he said.

Long said he could not comment in detail but said the Strahl case is significantly different than Lykken's.

"Different facts and different evidence," he said.