The Bureau was confident that those involved in the shocking Yosemite Park murders of sightseers Carole Sund, her daughter, Juli, and family friend Silvina Pelosso were already behind bars on unrelated charges.
Then came the death of Yosemite Park naturalist Joie Armstrong, whose decapitated body was found near her home in the park.
That slaying blew away the FBI's belief that the Yosemite killers were already behind bars. Subsequently, former motel handyman Cary Stayner Â— a man the FBI had interviewed and released the day after Armstrong was reported missing - was arrested. Stayner later confessed to killing the three sightseers and to murdering Armstrong.
However, officials close to the investigation told CBS News that Stayner's confession was not enough to charge the hansdyman with the murders. They need physical evidence that would corroborate Stayner's admission.
To that end, the FBI sent Stayner's knife to be examined for traces of the victims' blood and agents are still searching for Stayner's blood-soaked clothes.
By avoiding arrest during his first interview with federal officials, Stayner fooled the FBI. One senior official told CBS News that Stayner was "cooperative and calm" during the interview. Stayner has also been described by FBI sources as very articulate, intelligent and crafty.
Despite a deep sense of embarrassment on the part of the FBI for releasing Stayner and making people feel safe when they actually weren't, the FBI still has not ruled out the notion that more than one person may have been involved in the four murders.
"Mr. Stayner is a suspect in the murders," FBI spokesman Nick Rossi said. "At the same time, the possible involvement of other people is still being evaluated"
Rossi would not elaborate or provide specific details due to the "pending nature" of the investigation.
FBI officials previously had said they were investigating a group of methamphetamine users in Modesto, California, in connection with the deaths of the three sightseers. The group included Eugene "Rufus" Dykes and Michael "Mick" Larwick, half-brothers with a long history of weapons, drug and sex offenses.
Dykes and Larwick have both denied any connection to the murders. Although the FBI won't confirm it, federal agents may have targeted the group because the wallet of one of the victims was found in Modesto.
Some say Stayner confessed to all four murders to draw attention to himself Â— attention much like his brother, Steven, received when he escaped from a kidnapper who subjected him to years of sexual abuse.
Meanwhile, the FBI's decision to release Stayner after questioning him a day after Armstrong's disappearance continues to draw harsh criticism.
U.S. Sen. Charles Grassley, who heads a subcommittee that oversees the FBI, slammed the agency for its handling of the investigation.
The Iowa Republican compared the cas to that of Richard Jewell, a security guard who was the initial focus of the FBIÂ's investigation into the bombing at the 1996 Olympics in Atlanta, but was later cleared.
Â"Incidents like these by the FBI tend to undermine public confidence in federal law enforcement,Â" Grassley said in a statement.
Francis Carrington, father of Carole Sund, said, Â"Apparently somebody made a mistake, didnÂ't they?Â"