Police say around 2:00 a.m. Saturday, the 20-year-old victim, who was a passenger in a Toyota Camry, got into a road rage incident with four people in a dark sedan on Route 1 North.
It started on the road but ended in a parking area at the Biddles Toll Plaza.
Investigators say the 20-year-old, who is from Sussex County, jumped out of the vehicle and hit the rear window of the sedan, shattering it.
A fight broke out. Then someone from the sedan opened fire, striking the 20-year-old once in the chest.
The two groups took off in different directions. The victim and his friend stopped on the Roth Bridge.
"On my way home I had noticed the whole northbound Route 1 was shut down, hundreds of cops everywhere.... So I knew something bad had happened," said McKenzie Davis, who was driving by.
"I just got this gut feeling like this is something bad, and it sure enough was a person, and I couldn't believe it," Davis said, referring to the wounded 20-year-old man.
State Police investigators are looking at surveillance video. They spent a couple of hours Saturday morning collecting evidence, and now they are looking for tips from the public.
Police say the shooter and his friends were last seen heading north on Route 13 over the Saint Georges Bridge.
Anyone with information about the incident or the suspects is asked to contact Delaware State Police.