Details about the 2017 killings of two teenage girls in Delphi, Indiana, were revealed for the first time Friday during the trial of the man accused of killing them.

Richard Allen is charged with the murders of Abby Williams, 13, and Libby German, 14. The two friends were found dead a day after a walk on a hiking trail in February 2017.

Allen pleaded not guilty to four counts of murder.

Police have never revealed how the eighth-graders were killed. During opening statements Friday in the long-awaited trial in Carroll County, Indiana, prosecutors provided jurors with details about the murders.

“They will look at the crime scene,” said prosecutor Nick McLeland. “It was a cruel scene. Libby was completely naked. Her throat was cut and there was blood everywhere. Abby’s throat was also cut.”

McLeland said the case is about three things: the “Bridge Guy,” an unspent bullet found at the crime scene, and the brutal murders of Libby and Abby, who were found dead near the Monon High Bridge.

According to McLeland, Libby posted a photo on Snapchat of Abby as they crossed the Monon High Bridge. After the girls crossed the bridge, they saw a man behind them, so Libby started recording on her phone at 2:13 p.m. on Feb. 13, 2017, he said.

McLeland said the man pulled out a gun and ordered the girls to go “down the hill.” The girls obeyed and then the video stopped recording on the cell phone.

According to McLeland, Allen testified that he was on the trail that day. Investigators also found a gun in his home, and investigations revealed that a bullet found at the crime scene penetrated that gun.

McLeland said Allen also confessed to committing the crime against his wife and mother voluntarily while in prison.

Defense attorney Andrew Baldwin said in his opening statement that there was reasonable doubt in the case and argued that the state's investigation was botched from the start.

Baldwin questioned the timing and cell phone evidence in the state's case, holding up a phone to the jury and saying, “Forensic data on these phones doesn't lie.”

Baldwin said prosecutors allege that Abby and Libby were dead at 4 p.m. on Feb. 13, 2017, and their bodies were not recovered until the next day. He said the prosecution's timeline assumed Allen was in a parking lot near the trial at 1:30 p.m., but his cellphone records showed he disappeared at 2:15 p.m

Baldwin addressed the unspent cartridge found at the scene, saying law enforcement often used this type of bullet, but police never investigated whether an officer was missing one. He also said the owner of the property where the girls were found owned a similar weapon, but his weapon was never tested.

The defense also argued that Allen's mental health deteriorated in prison, leading him to confess to the crime.

The defense told jurors that they believe the girls were killed elsewhere and their bodies were returned to the crime scene – that searchers did not see any bodies or girls' clothing in the creek on the night of February 13 and that there were never any witnesses in near the crime scene I heard some screaming.

Baldwin also said that hair found in Abby's hand may match a female relative of Libby and not Allen. The defense disclosed the evidence for the first time during the trial earlier this week.

Before opening statements, Judge Frances Gull ruled that composite sketches of a person involved in the case released by the Indiana State Police early in the investigation could not be used during the trial.

Prosecutors had filed a motion to prevent defense attorneys from referring to the sketches, arguing that they were intended to generate leads in the case and would not be used to identify Allen as a suspect in the case.