And now a suspect has been arrested and charged with their murder.
On Monday, Indiana State Police announced they charged 50-year-old Richard Allen with the murders of German and Williams.
Here's what we know so far.
German and Williams went missing on 13 February 2017 after they set off on a hike along the Monon High Bridge Trail in their hometown of Delphi, Indiana.
After the girls did not return home a search went underway until their bodies were discovered approximately half a mile off the trail.
The girls' cause of death remained a mystery, however, shocking new details about the murders came to light in a search warrant application obtained by the podcast The Murder Sheet and shared with The Independent back in May.
The warrant revealed the young girls lost “a lot” of blood and their killer is believed to have moved and staged their bodies, before taking some sort of souvenir from the scene.
Although redacted, the warrant also revealed the girls' were murdered by some type of weapon.
While the two young teenagers were on their hike, German posted photos to her Snapchat and recorded video/audio of the hike.
In one audio clip obtained from German's phone, a man can be heard telling the girls "down the hill".
In a video clip, a man wearing jeans, and a jacket can be seen walking behind the girls on the trail. Investigators suspected the man in the video to be German and Williams' murderer.
A composite sketch of the unknown man was made with the description of being 5' 6" to 5' 10", weighing 180 to 220 pounds, and having reddish brown hair.
\u201c500 new tips in #Delphi murders since Monday's release of sketch of a person of interest. Tips 844.459.5786 or email@example.com\u201d
It's unclear what information led to the arrest of Allen as his name has never been publicly tied to the case.
Allen would have been 44 at the time of the murders, he is a resident of Delphi.
According to The Independent, Allen is a trained pharmacy technician working at a local CVS. German's grandparents, Mike and Becky Patty said that they remembered Allen processing photos for them in the store.
CVS offered condolences to the victims' families saying, “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," they added.
\u201cThis is a photo from #Delphi murder suspect Richard Allen\u2019s wife\u2019s Facebook. What\u2019s on the wall behind Allen is quite disturbing\u2026\u201d
— Christopher Duett (@Christopher Duett)
Allen is married to his wife, Kathy, together they share one daughter.
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