Originally Published: 29 JUN 23 09:07 ET

Updated: 29 JUN 23 10:40 ET

By Joe Sutton and Lauren Mascarenhas, CNN

(CNN) — Richard Allen, the man charged with the 2017 killings of two teenage girls in Delphi, Indiana, confessed to the crime during a phone call with his wife while in custody, a newly unsealed court document alleges.

Allen was arrested in October and faces murder charges in the killings of Abigail Williams, 13, and Liberty German, 14, whose bodies were found a day after they went for a hike along Delphi Historic Trails. He pleaded not guilty.

His arrest followed a five-and-a-half-year nationwide search for a suspect.

Details of the alleged phone call were included in a large number of documents unsealed Wednesday by Judge Frances C. Gull, the special judge who has been assigned to oversee the case.

On April 3, Allen called his wife, Kathy Allen, one document states. “In that phone call, Richard M. Allen admits several times that he killed Abby and Libby,” according to the document.

“Investigators had the phone call transcribed and the transcription confirms that Richard M. Allen admits that he committed the murders of Abigail Williams and Liberty German. He admits several times within the phone call that he committed the offenses as charged. His wife, Kathy Allen, ends the phone call abruptly,” the document went on to say.

Allen’s attorneys previously said their client “has nothing to hide,” and they planned to make “a vigorous legal and factual challenge” to the prosecution’s claims.

The investigation began after then teens went for a hike during a day off from school on February 13, 2017, and didn’t show up at a previously arranged time to meet Libby’s dad, police said.

Their bodies were found the next day in a wooded area near the trail, about a half mile from the Monon High Bridge where they’d been dropped off.

Abby posted a photo to Snapchat of the girls crossing the railroad bridge shortly before they were killed, police said. Indiana State Police investigators released a grainy, pixelated image of a man in a blue jacket and jeans on the bridge, along with a recording of a man’s voice. Investigators said they believed the man was a suspect in the double homicide.

A probable cause affidavit released by a judge in November revealed evidence that led investigators to believe Allen forced the girls down a hill and to the location where they were killed, including a .40 caliber unspent round from the scene that was tied to Allen, according to the affidavit.