Three teens accused of driving around and throwing large rocks at passing cars, one of which allegedly killed a woman, are said to have circled back to take a photo of her crashed car as a “memento”.

The three 18-year-olds were arrested at their homes in Denver, Colorado, after being identified as suspects with the help of cellphone tower data. All have been charged with first-degree murder.

Nicholas “Mitch” Karol-Chik told investigators that Joseph Koenig slowed down so Zachary Kwak could get a photo of the car where authorities say 20-year-old Alexa Bartell died after being hit in the head with a rock on April 19, according to arrest affidavits.

In a hint at a possible motive, Karol-Chik said all three got excited every time they hit a car with a rock that night but acknowledged he felt “a hint of guilt” after passing by Bartell’s car, according to the documents.

Nicholas "Mitch" Karol-Chik is facing a first-degree murder charge. Photo / AP

Kwak said he took the photo because he thought Karol-Chik or Koenig would want to have a “memento”, according to the affidavits. Koenig did not speak to investigators after he was arrested.

The three teens appeared briefly in court for the first time but spoke only to answer short questions from the judge about whether they could hear and understand what was happening.

A telephone message left for Kwak’s lawyer was not immediately returned. A person at the law firm appointed by the court to represent Karol-Chik declined to comment. Koenig is represented by a lawyer from the public defender’s office, which does not comment on cases to the media.

Zachary Kwak is facing a first-degree murder charge. Photo / AP

Joseph Bopp, a friend who had been hanging out with the teens earlier on April 19, offered a possible explanation for the alleged rock-throwing, telling investigators that Koenig often participated in “destructive behaviour” because “he likes causing chaos”, the documents said.

Bopp said he asked to be taken home after he saw the three others taking landscaping rocks from a Walmart parking lot and loading them into Karol-Chik’s pickup because he said he knew something bad was going to happen, according to the documents.

Karol-Chik told investigators that he and Koenig had thrown rocks and a statue at passing cars on at least 10 other days before Bartell was killed. Kwak heard about what they were doing and asked to join them on April 19, according to Karol-Chick, the documents said.

Investigators believe the teens were involved in several other similar incidents in which rocks weighing up to 2.7 kilograms were thrown at cars on the night of Bartell’s death. Two other drivers suffered minor injuries.

According to the sheriff’s office, Bartell was talking on the phone with a friend when she was hit by a rock. After the call went silent, the friend tracked Bartell’s location with a phone app and found her dead in her car, which had crashed into a field.

- Colleen Slevin, AP