Investigators had no way of knowing 32 years ago that 18-year-old Tammy Bristow held the key, under her fingernails, to identifying the man who investigators believe put the rope around her neck and caused her death inside her Sandpoint home.

Instead, Bristow’s killing on Jan. 8, 1987, had become a decades-long mystery after the case fell apart against former suspect Fred Gauerke, who provided several reasons for law enforcement to think he was the killer.

That apparent mystery ended Thursday when a grand jury returned an indictment against William R. Acosta, who was arrested in Ponderay Thursday afternoon on a charge of first-degree murder in connection with Bristow’s killing.

The case broke after Sandpoint police Chief Corey Coon authorized some of the evidence in the Bristow killing to be retested. Under Bristow’s fingernails, Idaho state lab technicians found DNA matching Acosta, who was convicted in 1988 of the armed robbery of a Sandpoint taxi driver, Bonner County Prosecutor Louis Marshall said during a news conference Friday.

“Obviously, we feel DNA under the victim’s fingernails is strong forensic evidence,” Marshall said. “Whenever you are dealing with a case like this, there are obviously some hurdles that we have to go through. Nineteen eighty-seven was a long time ago. We have to show chain of custody. We have to figure out who did what in terms of the evidence.”

When the killing occurred, responding officers found no signs of forced entry. That led them to believe the suspect may be familiar with the victim. Marshall said Acosta was known to Bristow. She had been babysitting the child of Acosta’s girlfriend, he said.

The motive for the killing had always been robbery, Sandpoint police detective Mike Aerni said.

“Ms. Bristow was paid some money earlier that day and that money was missing” when she was found at about 3 p.m., Aerni said. “The investigation led at the time to Gauerke, who was a local transient in the area. He was the primary suspect from now until this time.”

Marshall explained why investigators thought Gauerke, who is still alive but lives somewhere in the upper Midwest, was their killer.

“Mr. Gauerke was certainly a colorful character. In retrospect, there is no question why investigators at the time, and the prosecutor at the time, thought of him as the prime suspect in the case,” Marshall said. “His behavior was bizarre, to say the least.”

Some six days after Bristow was found dead, Gauerke “broke into a home of a woman in south Sandpoint and basically held her captive for 20 minutes,” Marshall said.

When investigators arrested Gauerke for the home invasion, he had Tammy Bristow’s bible in his possession, Marshall said. “That was strong evidence at the time.”

As part of the case that led to murder charges against Gauerke, investigators found fibers on his person that appeared to match fibers found inside Bristow’s home.

Gauerke spent 11 months in jail on the murder charge until FBI analysis showed that the fiber evidence “was not conclusive” enough to tie him to the murder scene at Bristow’s home, Marshall said.

At that point, former Bonner County Prosecutor Phil Robinson dismissed the charge, “which in retrospect seems like a very good move,” Marshall said.

Investigators to this day do not know how Gauerke ended up with Bristow’s bible, Marshall said.

“We have to essentially prove that Mr. Gauerke did not commit this crime,” he said. “So, that’s an added hurdle that we have here. I think we’ve been able to do that through forensic evidence … to exclude Mr. Gauerke and include Mr. Acosta.”

In addition to his robbery conviction from 1988, Marshall said Acosta has been arrested on the charges of vehicular homicide and aggravated assault in Phoenix, Arizona.

Acosta, who was scheduled to be arraigned Friday afternoon on the new murder charge, remains in the Bonner County Jail on a $1 million bond.

Marshall would not comment on whether Acosta has given any statements to detectives about what happened 32 years ago.

Aerni, the detective, said his department never gave up on the case.

“For the past three and a half years,” Aerni said, “this case became a priority for us, and we have been working on it since that time.”