The three suspects accused of throwing a large rock that killed 20-year-old motorist Alexa Bartell in Colorado last week pledged a “blood brothers” oath to keep their actions a secret, officials said. 

“Nicholas ‘Mitch’ Karol-Chik, Joseph Koenig and Zachary Kwak, all 18, have all been booked on suspicion of murder. Arrest warrant affidavits painted a grisly picture of 20-year-old motorist’s [sic] Alexa Bartell’s death on a lonely stretch of Indiana Street in Jefferson County, between Denver and Boulder,” NBC News reported on Thursday.

Jefferson County sheriff’s investigator Daniel Manka revealed in an affidavit that Bartell was talking on the phone with her friend Jenna Griggs at 10:45 p.m. on April 19 when she “suddenly stopped speaking,” according to the report.

Griggs used the Find My iPhone feature to locate Bartell, the report states. Griggs ultimately found Bartell inside her car, not moving, and with a significant head injury, according to the affidavit.

Griggs subsequently called Bartell’s mother and 911. When law enforcement arrived, Bartell did not have a pulse, according to the report.

“Investigators found ‘biological matter” throughout the car and a large “‘river rock’ landscaping rock” on the side of the road, stained with blood,” according to the report, which cited the affidavit. “Several other motorists that night reported stones were hurled at their cars around the same time in the area where Bartell was fatally struck.”

Sheriff’s investigators used cellphone data pinging off nearby towers to find one that “passed in the area of the death of the victim at the same time that the victim, Alexa, stopped talking,” documents detailed. Investigators were able to link a phone to Koenig and his mother Lara Koenig.

Detectives also spoke with Koenig’s friend, 20-year-old Joseph Bopp, who said he was with the three suspects at Walmart on April 19 when he saw them “picking up landscaping rocks from the edge of the parking [lot] and putting them in the back seat of ” Karol-Chik’s truck, according to the affidavit.

“Joseph [Bopp] states that he knew something bad was going to happen, so he insisted they take him home, which they did,” according to court documents.

Karol-Chik told detectives Kwak was the one who threw the stone that struck Bartell, and that Koenig drove the truck back to the scene to look at the damage, according to the report.

“Joe slowed the vehicle so that Zach could take a photo of it. Mitch noted that he felt ‘a hint of guilt,’” the court document states, according to the report. 

Kwak told detectives he took a picture of the wreckage because “he though Joseph [Koenig] or Mitch would want it as a memento,” the affidavit detailed, according to the report.

He also told detectives that “Joseph [Koenig] and Mitch were talking about them now being ‘blood brothers’ and they could never speak of this incident,” court documents allege.

The suspects also allegedly met up the next day to get their stories straight, according to the affidavit.

A judge ordered all three suspects held with out bail. Officials have not released any details about the motivation behind the rock-throwing incident that ended in Bartell’s death.