By Tim Jimenez and Alexandria Hoff

CHESTER COUNTY, Pa. (CBS) –– A manhunt is on for a suspect who authorities say gunned down a high school grad moments before her car veered off the road in Chester County on Wednesday.

Chester County District Attorney Tom Hogan announced on Thursday night that 18-year-old Bianca Nikol Roberson was shot in the head by a man driving a pickup truck

“A red pickup truck merged in at the same time and the man in the pickup truck pulls out a gun and shot her in the head killing her,” said Hogan.

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On Wednesday afternoon, police found Roberson inside her car lying in a ditch.

Investigators say that the pickup truck was seen next to Roberson’s car before the deadly encounter, it then sped off, entering Route 202, and continued to flee on the Paoli Pike.

“To that person, to that man, who fired that shot, turn yourself in now. Every second you are out there, you are only making this worse for yourself and making this worse for this young lady’s family,” Hogan said.

Roberson’s many friends describe her as loving and full of laughter.

She had just graduated from Bayard Rustin High School in West Chester, two weeks go with plans to attend college.

“This is a time of joy and celebration in a young person’s life and for her family, and it has been replaced by just utter sadness and tragedy,” said a school spokesperson.

Counselors were on hand Friday morning at Rustin High School to help students deal with the sudden loss of the recent graduate.

Pennsylvania Crime Stoppers is offering a 5,000 reward to anyone with information leading to an arrest or conviction in this crime.

A GoFundMe has been set up to help the Roberson family with burial costs.