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

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

Allen has pleaded not guilty to four counts of murder.

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

"You're going to see the crime scene," prosecutor Nick McLeland said. "It was a gruesome scene. Libby was completely naked. Her throat was cut, blood all over. 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 of Abby on Snapchat while they were crossing over the Monon High Bridge. After the girls crossed the bridge, they saw a man behind them, so Libby started a 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 "down the hill." The girls complied and then, the video on the cellphone stopped recording.

According to McLeland, Allen testified that he was on the trail that day. Investigators also found a gun in his house, and testing showed a bullet found at the crime scene cycled through that gun.

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

Defense attorney Andrew Baldwin said in his opening statement that there is reasonable doubt in the case, arguing that the state's investigation was botched from the beginning.

Baldwin questioned the timeline and cellphone evidence in the state's case, holding up a phone to the jury and saying, "Forensic data on these phones don't lie."

Baldwin said the prosecution claims Abby and Libby were dead by 4 p.m. on Feb. 13, 2017, and their bodies were never moved until they were recovered the next day. He said the prosecution's timeline puts Allen in a parking lot near the trial at 1:30 p.m. but his cellphone data shows he was gone by 2:15 p.m.

Addressing the unspent round found at the crime scene, Baldwin said law enforcement commonly used that type of bullet, but police never investigated if an officer was missing one. He also said the owner of the property where the girls were found owns a similar weapon but his gun was never tested.

The defense also argued Allen's mental health was in decline while in prison, which led to him to confess to the crime.

The defense told jurors they believe the girls were killed somewhere else and their bodies were returned to the crime scene -- that searchers saw no bodies or girl's clothing in the creek on the night of Feb. 13, and witnesses near the crime scene also never heard any screams.

Baldwin also said hair found in Abby's hand was a possible match to a female relative of Libby, and not Allen. The defense revealed the evidence for the first time during proceedings earlier in the week.

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