Richard Allen, a 50-year-old Delphi man, was arrested last month.
DELPHI, Ind. -- Evidence in the Delphi, Indiana, double murder case was unsealed by a county court on Tuesday, revealing key new details, including that suspect Richard Allen's gun was linked to the crime scene.
But many questions still remain around the murders of Abby Williams, 13, and Libby German, 14.
Here's what we've learned and what remains unclear:
Abby and Libby, best friends in the eighth grade, were on a hiking trail in rural Delphi when they were killed in February 2017.
Allen, a 50-year-old Delphi man, was arrested last month. He's charged with two counts of murder and has entered a not guilty plea.
When interviewed by police in 2017, Allen said he was on the trail on the afternoon of the murders, according to the probable cause affidavit.
WATCH: Police release video of Delphi murder suspect
In an Oct. 13, 2022, interview, Allen told police he saw juvenile girls on the trails east of Freedom Bridge and said he went onto the Monon High Bridge, near where the girls were killed.
This year, Allen "again admitted" to police "that he was on the trail but denied knowing Victim 1 or Victim 2 and denied any involvement in their murders," according to the probable cause affidavit.
Allen "has been consistent" in police interviews over the years, former FBI agent and ABC News contributor Brad Garrett said. "He put himself at the scene, on the bridge."
But Garrett said he doesn't understand how it took so long for an arrest.
"In a small town, in a place where there's a small amount of traffic on this abandoned railroad bridge ... your suspect pool is fairly small," Garrett said, so police likely concentrated their investigation on Delphi and the surrounding communities.
According to video recovered from one of the victim's phones, Abby or Libby mentioned "gun" as a man approached them, the probable cause affidavit said.
A .40-caliber unspent round was found less than 2 feet away from one of the girls' bodies, and that unspent round went through a gun that Allen owns, according to the probable cause affidavit.
Garrett explained that an "unspent bullet is one that has the casing and the projectile still together." To get that, he said one of two things happens: 1.) Someone tries to fire the gun but it's a faulty bullet and it doesn't fire, or 2.) The gun jammed, which Garrett said is common.
During a search of Allen's home on Oct. 13, 2022, officers found knives and guns, including a "Sig Sauer, Model P226, .40-caliber pistol," the probable cause affidavit said.
Indiana State Police's analysis of Allen's gun "determined the unspent round located within two feet" of one of the victims "had been cycled through Richard M. Allen's Sig Sauer Model P226," the probable cause affidavit said.
"When asked about the unspent bullet, [Allen] did not have an explanation of why the bullet was found between" the girls' bodies, the probable cause affidavit said.
When Allen voluntarily spoke to police on Oct. 26, 2022, he said he never allowed anyone to borrow that gun, which he said he owned since 2001, the document added.
Garrett said he doesn't understand why it took police so many years to match an unspent round from the crime scene to a gun owned by a man who lives in Delphi.
Garrett said he hopes investigators went to all of the local gun stores to see their records of sales of .40-caliber-type weapons. Garrett said he's solved homicide cases that way, because typically a perpetrator buys a gun legally near his or her home, he said.
While it's unclear if police did go to gun stores, Garrett think it's unlikely because there was no mention of a gun in the case until the probable cause document was released Tuesday.
Despite mention of a gun, it's not clear if Abby or Libby died from gunshot wounds. Police still have not released their causes of death.
The probable cause affidavit did reveal that clothes belonging to the girls were found in a creek south of where their bodies were discovered.
"I've always been concerned about how these two youngsters died. The police have put a .40-caliber weapon into the case," Garrett said. "You have this unspent shell casing near the victims' bodies, but you also have things that are really troubling to me: [The girls] are in one place and their clothes are in another. ... Unless he made them undress -- which I guess is possible -- was there some other weapon used?"
Investigators also cite a witness who saw Allen walking with "clothes that were muddy and bloody," according to the probable cause affidavit.
According to Garrett, it's unlikely Allen would be bloody if a gun was the only murder weapon, unless Allen handled the bodies in some manner.
Garrett said it's possible that the gun jammed and the killer turned to another weapon.
Knives were also found at Allen's home, according to the affidavit.
"Why would the police withhold [the cause of death]? The only thing I can think of is it was too gruesome, in their mind, to release," Garrett said. "It seems like there is something more to it than just a gun."
Video from one of the victim's phones shows a man on the trail wearing dark jacket and jeans. An image taken from the video was released years ago as police asked for information to help them find the unknown suspect.
Investigators said in the probable cause affidavit that they believe Allen is the man seen on the video.
Allen told investigators on Oct. 13, 2022, that he wore jeans and a blue or black Carhartt jacket that day, according to the probable cause affidavit. Allen's wife confirmed to police that he owns a blue Carhartt jacket, the document said.
Investigators also claim Allen forced Abby and Libby down the hill to the spot where they were killed, according to the document.
Allen's lead defense attorney Brad Rozzi did not respond to a request for comment and fellow attorney Andrew Baldwin declined to comment.
Indiana State Police told ABC News on Tuesday: "Out of respect for the prosecutorial process, which is being led by the Carroll County prosecutor, we are refraining from making any public statements and are going to allow the probable cause affidavit to stand on its own. As this continues to be an active and ongoing investigation, the Indiana State Police will continue to provide any and all resources available to assist in the prosecution of this case."
Carroll County Sheriff Tobe Leazenby said the information in the probable cause affidavit is "self-explanatory" and declined to comment further.