"They jockeyed for position and he wasn't happy, so he pulled out a gun and shot Bianca in the head, killing her instantly", Hogan said. Desper complied with orders, and turned himself in to police on Sunday at 2 A.M. EST.
"This is a story of a savage of senseless murder."
District Attorney Tom Hogan said investigators believe the driver may be from the area because of "where they were driving and that time of day, and where we could track the truck going".
Responding police found Roberson's green Chevy Malibu in a ditch as it apparently veered off the road after the incident.
Roberson had just graduated from Bayard Rustin High School in West Chester and was planning to attend Jacksonville University in Florida this fall.
He said there was no indication that race played a role in the murder - Roberson was black and Desper is white.
He is set for a preliminary hearing July 13.
West Goshen Police Chief Joseph Gleason described the shooter as a 20- to 40-year-old man with blond hair and a medium build.
They also confiscated the red pick up truck he was driving when he encountered the teenager at the interchange of Route 100 and Route 202. After the shooting, Roberson's vehicle went off the highway and struck a tree.
Hogan was joined by West Goshen police at a Sunday morning press conference to announce that an avalanche of tips from the public led first to the red pickup truck that was the subject of a nationwide search, then to the gun used to end a young life, and then to the suspect.
Roberson's death is the latest tragedy for her family; the teen's 22-year-old brother died from heart disease a few years ago.
She had been at the mall shopping for college clothes with her grandmother and mother when she was killed as she made her way home. "And then when she saw the police she said she had this insane feeling".