Twenty-three-year-old Jeremy Webster told police he started taking a new prescription medication for mental health issues Thursday, just hours before he settled a driving dispute with his Glock 19 handgun, according to a search warrant affidavit.

The road-rage encounter, which started at a busy street corner and ended at a Westminster office complex, took no more than 10 minutes.

Thirteen-year-old Vaughn Bigelow is dead. His 41-year-old mother, Meghan, and 8-year-old brother, Asa, are in critical condition at a hospital. A bystander, John Gale, is recuperating from gunshot wounds to both arms.

Webster is being held without bond at the Adams County Detention Facility for investigation of first-degree murder, criminal attempt to commit murder with extreme indifference and first-degree assault.

Before Thursday’s incident, Webster, who lives in Colorado Springs, had never been charged with a crime in Colorado and had never seen the four people he is accused of shooting, Westminster police say.

At around 3 p.m., Webster and Meghan Bigelow had some type of angry encounter in traffic near Sheridan Boulevard and Turnpike Drive. Bigelow was driving with her three sons — the third was Cooper, 12 — in the car.

Webster allegedly followed the Bigelows in his Toyota Corolla as she drove about a mile north on Sheridan and pulled into the complex of commercial stores and offices. Bigelow parked in front of Children’s Dentistry, 5250 West 80th Ave.

She evidently knew she was being followed. Cooper would later tell police that his mother pulled out her cellphone and took a picture of her pursuer’s license plate, NPQ091. She and her boys climbed out of their car at 3:04 p.m. Several people, including Cooper, would recount what happened next, according to the affidavit.

A thin man — with dirty-blond hair and wearing a maroon shirt and khaki pants — stopped his black Toyota in the middle of the parking lot, climbed out and immediately confronted Bigelow and her sons. A short, heated argument ensued.

The man suddenly pulled out a handgun and shot Meghan Bigelow repeatedly. When she fell to the pavement, the man aimed his gun at Vaughn and Asa and shot each of them multiple times. The man then began walking to his car. He then turned around, walked back to the family and again shot Vaughn, who was lying on the ground, the affidavit says.

Only Cooper managed to run to a building and get inside without being wounded.

The shooter then looked over at Gale, who was sitting with his 9-year-old daughter in his truck. The man walked toward Gale and started shooting at him as he put his truck in reverse and sped out of the parking lot. Gale’s daughter was not injured.

Webster then climbed back into his car and drove away. Cooper later told police that his mother had taken the picture of the shooter’s license plate. Police recovered Meghan Bigelow’s cellphone, put out a bulletin with a description of the suspect’s car and the license plate number.

About three hours later, authorities arrested Webster near Castle Rock while he was apparently driving home on Interstate 25. In a Westminster police interview room, Webster told police that his Glock handgun was in the trunk of his car and his prescription drugs were at home.

“Jeremy eventually admitted that he was the one who shot Meghan, both of her children and John Gale,” the affidavit says.

Webster was booked into the Adams County jail at 1:39 a.m. Friday, more than 10 hours after the shooting.