A witness who was at the Seahawks facility during Tuesday's crash says Fred Jackson was revving his engine at another driver before both cars rocketed down a 1 mile stretch ... leaving no doubt they were drag racing.
We spoke with Jason Hellervik -- an autograph seeker who was waiting for players at the facility when he saw Fred's black Corvette pull up to a stop sign next to a red Dodge Charger.
"They were both stopped at the same time, they both revved their engines and they both left without stopping," says Jason, who was with his buddies at the time ... "We were like what are these idiots doing?"
Hellervik says he was with 6 other guys at the time of the crash -- who all witnessed the apparent drag racing scene -- and none of them have been contacted by police. He says if contacted, he would definitely cooperate with investigators.
For his part, Jackson says he was not racing anyone.