The bullet was fired by a man driving a red pickup truck on a Chester County highway last week, police said, striking and killing 18-year-old Bianca Roberson as she tried to merge lanes next to him.

And as authorities announced murder charges Sunday against the man who allegedly pulled the trigger, they said the bullet was a key piece of evidence in securing his arrest, because the fragment later pulled from Roberson's head during an autopsy matched the caliber of gun they found at his Delaware County house.

"This is the story of a savage and senseless murder," said Chester County District Attorney Tom Hogan. "Somebody didn't want to give way. Somebody didn't want to merge into a lane of traffic. And because of that, a young woman is dead."

David Desper, 28, of Trainer, surrendered to police around 2 a.m. Sunday, Hogan said, ending a three-day manhunt for the road-rage murder suspect that spanned at least two counties and attracted national attention.

Roberson's family sat in the audience as Hogan and a team of police officials in West Goshen Township, where the slaying occurred, announced the charges against Desper, including first- and third-degree murder, possession of an instrument of crime, and reckless endangerment. The family did not want to speak afterward, and they were visibly distraught during the news conference, shaking their heads as the district attorney recounted the facts of the case.

Hogan said Roberson — a recent graduate of Bayard Rustin High School in West Chester who was headed to  Jacksonville University in the fall — had "her whole summer in front of her. Her whole life in front of her."

Her father, Rodney Roberson, said last week that his daughter, who also worked at a retirement community in Newtown Square, was shot while driving home from a shopping trip in Exton with her mother and grandmother, picking up school supplies and groceries.

Police had been searching for leads since the killing Wednesday evening on Route 100. Hogan said investigators interviewed witnesses, collected video surveillance, and released to the public pictures of Desper's truck and a composite sketch of the suspect's face — an attempt to force the alleged killer to turn himself in.

The gambit ultimately worked, with Desper surrendering at the Media office of his lawyer, Dan McGarrigle.

McGarrigle declined to say Sunday why his client wanted to turn himself in or whether he had given a statement to authorities. Hogan also would not say whether Desper had told police anything about the crime.

Several of Desper's neighbors in Delaware County said they did not know him well; some said they would not have recognized him. No one answered the door at his home on Anderson Avenue, though Mark Halliday, 38, who lives two doors down, said he believed two or three men moved into the house about two years ago.

Fred Kinsler, 45, said he had known Desper as a kid. Kinsler knew that Desper drove a red truck but said he "didn't put two and two together" until he heard about the arrest.

"He was a good kid," Kinsler said. "It's a shame."

Hogan said Desper worked as a well and pipe driller for local businesses, though he declined to elaborate. He said Desper's parents were living with him at the house and were there when authorities searched it last week.

Attempts to reach Desper's family Sunday were unsuccessful.

6ABC's Annie McCormack reported finding this sign in a home said to belong to relatives of Desper.

Hogan suggested that the motive for Desper's actions was relatively cut-and-dry.

Desper and Roberson "jockeyed for position [while merging] and he wasn't happy," Hogan said. "So he pulled out a gun and shot Bianca in the head, killing her instantly."

"That's the same merge people in Southeastern Pennsylvania make thousands of times every day," Hogan said. "All across the region people do that without a problem."

The district attorney said there was "no indication that we're aware of that this was a race crime or a hate crime."

"This appears to be a savage, senseless, and brutal act from one human being to another human being," he said.

The shooting happened at 5:31 p.m. Wednesday in the southbound lane of Route 100 approaching Route 202, according to police. Roberson was driving a green 2009 Chevrolet Malibu, Desper a red Chevrolet Silverado pickup truck.

After Desper shot Roberson in the head, police said, her car veered off the road and crashed into a tree. One witness told police that just before seeing the crash, he "heard a loud noise that could have been a gunshot," according to a criminal complaint.

Hogan said surveillance footage showed that Desper's truck sped from the scene. Investigators "recovered a large amount of video evidence" showing Desper's truck traveling on Route 100, Route 202, Paoli Pike, and ultimately to Delaware County, the criminal complaint says.

After Desper turned himself in at about 2 a.m. Sunday, the complaint said, police recovered his pickup truck on Huntingdon Farm Drive, in Glen Mills. Hogan said that Desper was the truck's registered owner, and that it bore the same dent that could be seen on the truck in the surveillance tapes.

Police also searched Desper's home, the complaint said, and found a .40-caliber Smith & Wesson semiautomatic handgun, as well as .40-caliber ammunition in a trash can. Ballistics testing on the bullet fragment pulled from Roberson's skull indicated that the same caliber weapon and ammunition were used in her killing, according to the complaint.

Hogan said Desper legally purchased the handgun in November 2015 and had a permit to carry it.

West Goshen Police Chief Joseph Gleason credited teamwork between his officers and county detectives, saying he was proud of their cooperative work — as well as the response of the community. Alerts pushed out by police and the media resulted in hundreds of leads and tips, he said.

But in a somber note, the police chief took time to offer condolences to Roberson's family — who were struggling to cope with the sudden loss of their bright, young girl.

"This is such a tragedy that no one should have to endure," he said.