Mystery Colorado Highway Shooting on I-25 Points to Possible Serial Attacker

— -- After four attacks centered on a northern Colorado highway within three days, authorities are concerned a serial shooter may be on the loose.

Police are trying to determine whether the incidents in Larimer and Weld counties are linked, the most severe of which was Wednesday, when a woman was shot in the neck.

The 20-year-old woman was driving alone on Interstate 25 in Larimer County late Wednesday night 25 when she was shot and the front driver's side window of her car shattered, the Larimer County Sheriff's Office said.

She said in a call to 911, "I'm on the highway right now and somebody just hit me and I'm bleeding from my neck and I'm scared."

The 911 operator tells her to place a dry cloth on the wound.

The woman says, "I'm trying but it's on both sides of my neck. ... I'm just scared."

She underwent surgery and is expected to survive, according to Larimer County Sheriff Justin Smith. "Listening to how calm she remains with the dispatcher, I can tell you that she is a very brave young woman," Smith said.

While authorities believe the woman was randomly selected, they are concerned the shooting may not be an isolated incident.

The Larimer County Sheriff's Office cited a Tuesday morning incident when deputies were driving a department van on I-25 in Weld County, transporting three inmates from one jail to another, and the right passenger window shattered. No injuries were reported, the sheriff's office said.

Two more drivers also reported shattered windows.

A man was driving in Larimer County near I-25 Wednesday night when his car's rear window shattered, ABC affiliate KMGH-TV in Denver reported, and Thursday afternoon, a man was driving in Weld County near the highway when his driver's side window shattered. Neither man was injured, KMGH reported.

But David Moore, a spokesman for the Larimer County Sheriff's Office, said, unlike the Wednesday night shooting, these three incidents have not been “proven to be cases where windows were broken by a bullet."

Separately, an abandoned truck was also been found along I-25 with its back window blown out, according to KMGH. But Moore said it is too soon to connect the truck to the other incidents.

No suspects have been identified and the investigation is ongoing. The authorities urge drivers to report any suspicious behavior.

ABC News' Ryan Smith contributed to this story.