ABC affiliate WRTV in Indianapolis reported Allen is said to have admitted guilt in the murders "as many as 61 times during his months at Westville Correctional Facility."
"The statements given by the defendant were unsolicited by any of the individuals and were voluntarily given without coercion or interrogation," Special Judge Fran Gull wrote in the ruling. "The evidence shows he specifically sought out the Warden by written communication he initiated, and verbal statements he offered to the guards, inmates, mental health professionals, and medical personnel."
Allen was set to go to trial in May, but a judge delayed it until Oct. 14 through Nov. 15.
Allen, 51, who had worked and lived in the Delphi area, was arrested in October 2022 and charged in the 2017 slayings of 13-year-old Williams and 14-year-old German. He's pleaded not guilty to murder charges. In March, Gull approved two additional murder charges against Allen.
The killings have haunted Delphi, a city of about 3,000 where Allen worked at a drugstore, and the case has been closely watched in the state and nationwide.
Gull issued a gag order in December 2022 barring attorneys, law enforcement officials, court personnel, the coroner and the girls' family members from commenting on the case to the public or the media in any form, including social media.
The Associated Press contributed to this report.