A Delphi, Indiana, man, Richard Allen, has been arrested for the 2017 murders of eighth graders Abby Williams, 13, and Libby German, 14, Indiana State Police Superintendent Doug Carter announced at a news conference Monday.

The 50-year-old was arrested Friday and has been charged with two counts of murder, Carter said.

If anyone else was involved, that person will be held accountable, Carter vowed.

Carter said "today is not a day to celebrate," but called the arrest a "major step."

"This investigation is far from complete," Carter said.

The probable cause affidavit has been sealed, Carter said.

"The time will come when additional details can be released," Carter added.

In 2017, authorities released a grainy image of the suspect, who they say was on the trail the day the girls went missing. In 2019, police released a brief video clip -- footage taken from Libby's phone -- showing a grainy image of the suspect walking on the bridge near where the girls were last seen.

Police also publicized the suspect's voice -- a recording of him saying "guys ... down the hill" -- which was recovered from Libby's phone.

Authorities in 2019 released a new suspect sketch that officials said was based on a witness' recollection of what he or she saw.