Gwinnett County police are searching for a man who shot a 15-year-old Buford High School student in the head and killed him early Monday.

Officers were called to Gas Plus in the 4500 block of Thompson Mill Road after learning Branden Gerena was wounded behind the Buford-area business, police said in an emailed statement. He was taken a hospital, where he died.

Gerena just completed his freshman year at Buford High, according to a brief statement from Buford City Schools.

Investigators learned Gerena and his friend were at the gas station when a man in his 20s approached them and opened fire just after midnight. The friend, also 15, was not hit. It’s not clear why the teens were at the gas station or why the man started shooting.

The gunman ran after the incident and has not been located. He was wearing blue jeans, a black jacket with red lettering and a backward baseball hat at the time of the shooting, police said.

Authorities are knocking on doors in a neighborhood off Thompson Mill Road, looking for witnesses and searching for more evidence, according to Channel 2 Action News. Investigators have video of the teens hanging out in front of the gas station, the news station reported. The video shows them walk to the back of the business, where there are no surveillance cameras.

Anyone with information is asked to contact Gwinnett police at 770-513-5300. Tipsters can remain anonymous, and be eligible for rewards of up to $2,000, by contacting Crime Stoppers Atlanta at 404-577-8477, texting information to 274637 or visiting the Crime Stoppers website.