JEFFERSON COUNTY, Colorado -- 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 seven 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. On Saturday, investigators said they believed there may have been more than five incidents and asked victims to come forward.
On Monday evening, deputies reported two more victims came forward to the sheriff's office, saying a large rock struck them on April 19 at about 10:30 p.m.
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.
Now, investigators said they are looking for a light-colored pickup or SUV.
Based on a timeline released by the sheriff's office, the incidents began at 10:04 p.m. on April 19 when the suspect or suspects threw a rock through a car's windshield in Westminster. Authorities said 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 and injured the driver.
The next two incidents happened at 10:30 p.m. and 10:31 p.m. near the Rock Creek neighborhood, ten miles from the second incident in Arvada. The incident at 10:30 p.m. injured the driver after a rock broke the car's windshield. The incident at 10:31 p.m. damaged the body of a Toyota SUV, though authorities said the driver was unscathed.
The event that resulted in Bartell's death was the fifth such recorded incident Wednesday evening. At 10:45 p.m., Bartell was driving north on Indiana Street in Jefferson County when a large rock broke her windshield and killed her. The deadly incident was roughly two miles from the third and fourth incidents of the night.
The two new reports reportedly happened on Highway 93 between Highway 128 and Highway 72.
Police said the suspect was traveling northbound at about 10:30 p.m. when they allegedly threw the rocks at two cars, damaging one driver's side rear windows and the other motorist's windshield. No passengers in the vehicles were injured, according to the police.
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.
Bartell's employer, Commercial Flooring Services, announced Friday that it's offering a $15,000 reward for information about the homicide. On Monday, authorities announced an increased reward of up to $17,000.
"CFS lost our youngest, brightest employee to this senseless act," the company wrote in a social media post.
Vehicles with cameras and residents are being asked to check their surveillance footage for a light-colored pickup truck or SUV.
Authorities urge you to contact the Jefferson County Sheriff's Office at (303) 271-5612 or Metro Denver Crime Stoppers at (720) 913-7867 directly if you have any information related to this investigation.
Information leading to the charging and/or arrest of any felony suspects may result in a reward of up to $17,000. Tipsters must contact Metro Denver Crime Stoppers directly to remain anonymous and to be considered for a reward.
ABC News' Alex Stone contributed to this report.