CHESTER, Calif. -- Some type of sexual assault may have happened before or after the killings of two teens on their prom night, Plumas County investigators said.

Bloody evidence is described in a search warrant police obtained for the 18-year-old suspect in the case.

Reyes Carrillo Garcia's ex-girlfriend, Jennifer Carrigan, and her prom date, Steven Furtado of Willows, were found dead on Mother's Day at her home in Chester.

In the warrant, various items at Garcia's home in the 700 block of Watson Road were found stained in blood, including boxer shorts, a plastic shopping bag, a jacket and a napkin.

According to a statement of probable cause, both victims were found naked in a bedroom, and Furtado had a clean knife in his hand.

Garcia admitted driving to Carrigan's house that night, according to the document, and he parked his car about one block away.

He said he came in the house unannounced and heard Carrigan's voice in distress.

Garcia then went into a bedroom and saw Furtado and Carrigan having sex on the floor.

He told investigators he remembered "picking up some sort of object and engaged in physical contact, 'collided' with Steven," according to documents from the case.

Garcia told investigators he didn't remember anything else other than being back at his vehicle.

Sgt. Greg Hagwood said a used, bloody condom with what appeared to be bodily fluid inside was found about one block from the victim's residence in the 100 block of Aldon Drive, about one block from where Garcia said he parked his car, documents said.

Carrigan's and Furtado's keys were found between two railroad rails about 20 feet from the condom, according to a statement of probable cause.

A bottle of lubricant with a bloody smear was also found at the crime scene, documents said.

In addition, investigators found a bag in Garcia's back yard that contained a basket that was purchased at Bath and Body Works in Chico. Dan Furtado, Steven's father, said his son had mentioned on the phone that he had bought a Mother's Day gift on his way to Chester that day.

A receipt taken from Furtado's wallet as evidence was used to confirm with the store that he had purchased the basket.