Richard Allen is accused of killing teenagers Abby Williams and Libby German near a hiking trail back in 2017.
The trial started with two witnesses called by the prosecution.
The first witness was Carroll County Deputy Darren Giancola. He was the first law enforcement officer on the scene after the bodies of Libby German and Abby Williams were found on February 14, 2017.
Giancola said he was off duty the night the girls went missing but went into work about 10 p.m. on February 13th after seeing the case on social media.
As he testified, he got emotional while describing the condition the girls' bodies were found in.
"One was nude, the other was clothed. Both had large lacerations on their throat. They both had a substantial amount of blood on their person and underneath," he said.
The second witness to take the stand was Jason Page. Page works for Indiana State Police in the Crime Scene Investigation unit.
He was responsible for photographing the crime scene and the girls' bodies.
The graphic pictures shocked the court room and the jury. Some photos showed a close up of Libby's slashed throat and bloody face.
The jury was clearly uncomfortable with seeing the images and there were audible gasps in the courtroom.
The families of Libby and Abby were also in the gallery crying.
Testimony is starting back up and is expected to continue until about 6 p.m. Monday.
Over the weekend, jurors met on Saturday for a half day in the courtroom. On Sunday they were allowed to see their families.
ABC News contributed to this report.