Allen was taken into custody last week, one week after investigators searched his home in Carroll County.
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Though the case remains under a rare court-ordered seal, felony murder charges were listed in the state's tracking system.
Allen has pleaded not guilty. His trial is scheduled for March 2023.
Indiana State Police superintendent Doug Carter finally announced charges against 50-year-old Allen on Halloween.
Indiana State Police announce charges in Delphi murders
New details released on arrest in Delphi murders
"I had always told everybody that when there's an arrest, you're going to hear me across the country. But it just didn't feel that way," said Becky Patty, German's grandmother. "It's the beginning of a final chapter, I guess."
Police said Allen is from Delphi, and would have been familiar with the area around the Monon High Bridge where the girls went hiking on Feb. 13, 2017 and were found dead a day later on Feb. 14.
"That's going to take a while to process, but we're going to get through the trial and we'll have more to say," Supt. Carter said.
"This community has been scared. He's been walking among them just unknown," said James Wright, who lives in Lafayette.
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In an eerie twist, a photo of Richard Allen shows him at a local bar he frequented, sitting in front of one of the police sketches of the killer on the wall behind him.
SUSPECT AUDIO RELEASED | Police release video, sketch and audio of suspect in Delphi double murder
Police release new video of Delphi, Ind. suspect
Allen worked at a CVS in Delphi, the company said. In a statement, CVS said, "As members of the Carroll County community, we remain devastated by these murders and our hearts go out to the German and Williams families. We are shocked and saddened to learn that one of our store employees was arrested as a suspect in these crimes. We stand ready to cooperate with the police investigation in any way we can."
ABC7 has also found he had a limited criminal history.
The high-profile murder case has generated mountains of online speculation about who killed the two girls.
One key piece of evidence was audio clips and photos from Libby's phone. In the recording, a man, assumed to be the suspect, instructed the girls to go, "down the hill."
Delphi murders: Is it common for prosecutors to keep charging details sealed?
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Delphi murders: Is it common for prosecutors to keep charging details sealed?
Police had also released two different sketches of potential suspects over the past five years. There were many instances where thousands tips would come in authorities thought they had a break in the case. But, it never amounted to anything, until Monday.
"It was a very personal moment for me and I think it was a very personal moment for them, for all of us that have had interaction with the family," Indiana State police Superintendent Doug Carter. "We developed a relationship that I think will last at least one lifetime and maybe a second. It was a very, very yet humbling experience."
Libby's sister Kelsi tweeted, "just know how grateful I am for all of you."
"We've never forgotten them," Delphi resident John Rowe said. "Everybody's going to keep them in their minds forever."
In the meantime, police said they are still keeping the tip line open and emphasized that their investigation is not closed. They welcome any and all further tips, Indiana State Police said.
Police have not ruled out someone else possibly being involved in the murders.