20-year-old fatally struck by large rock while driving, 1 of 7 similar incidents

Alexa Bartell spoke to a friend over the phone while driving Wednesday night when the line suddenly dropped.

Concerned about Bartell's well-being, her friend eventually tracked Bartell's phone to a field in Jefferson County, Colorado, where she found a damaged yellow Chevy Spark. According to the Jefferson County Sheriff's Office, a large rock had broken through the car's windshield, striking and killing the 20-year-old Bartell -- one of at least five similar incidents in Colorado over three hours on Wednesday evening.

"This is the most tragic of a series of similar crimes that happened overnight throughout Jefferson and Boulder counties," the press release noted about the homicide.

An impromptu memorial site near the site of the homicide grew on Friday, with friends and family remembering Bartell.

"She was a friend to everyone and brought so much joy into everyone's lives," Bartell's friend told ABC News' Denver affiliate KMGH.

The Jefferson County Sheriff's Office has been unable to identify any suspects associated with the crime spree, including the homicide. Investigators also believe there may have been more than five incidents and have asked victims to come forward.

Law enforcement initially believed the incidents were associated with a 2003-2005 Dodge Ram pickup truck and asked the public to help identify its owners. However, investigators later confirmed the owner of the car was not "involved in the crime series related to the death of Alexa Bartell," leaving no suspects.

"[Jefferson County Sheriff's Office] continues to urge anyone with information about the crime to call our tip line at 303-271-5612," the press release read.

Based on a timeline released by the sheriff's office, the incidents began at 10:04 p.m. Wednesday when the suspect or suspects threw a rock through a car's windshield in Westminster, CO. The driver was uninjured by the incident.

At 10:26 p.m. nearly nine miles from the first incident, the suspect or suspects threw another rock through a windshield in Arvada, CO, injuring the driver.

The next two incidents occurred at 10:30 and 10:31 p.m. near the Rock Creek neighborhood, ten miles from the second incident in Arvada, CO. The 10:30 incident injured the driver after a rock broke the car's windshield, while the 10:31 incident damaged the body of a Toyota SUV, though the driver was unscathed.

The event that resulted in Bartells death was the fifth and final such recorded incident Wednesday evening. At 10:45 p.m., Bartell was driving north on Indiana Street in Jefferson County, CO, when a large rock broke her windshield and fatally wounded her. The final incident was roughly two miles from the third and fourth incidents of the night.

An earlier timeline released by the Sheriff's office stretched the incidents out over more than two hours Wednesday evening; the revised timeline narrows the crime spree to a 40-minute window across multiple miles. Investigators are still working to determine if the suspect or suspects threw the rocks from the side of the road or a vehicle.

"We believe there may be more victims," the sheriff's office's release noted, urging the public to come forward if they have any information or dash camera video from the evening that might offer insights into the deadly incident. With incidents crossing multiple counties in the Denver metropolitan area, the Jefferson County Sheriff's Office is working with the Boulder County Sheriff's Office, Arvada Police, and Westminster Police.

On Monday evening, the police revealed more information about an incident where the suspect allegedly threw rocks at two cars on Highway 93 between Highway 128 and Highway 72.

The suspect was traveling northbound around 10:30 p.m. on Wednesday when they allegedly threw the rocks at the cars, damaging one car's driver's side rear windows and the other motorist's windshield, police said. No passengers in the vehicles were injured, according to the police.

Bartell's employer, Commercial Flooring Services, announced Friday that it's offering a $15,000 reward for information about the homicide.

"CFS lost our youngest, brightest employee to this senseless act," the company wrote in a social media post.

ABC News' Alex Stone contributed to this report.