Tony Stewart Case Kevin Ward Was High On Weed ... Officials Say

Officials say Kevin Ward Jr. was high on marijuana at the time of his death. 

Officials say the "enhanced" video did not show Tony made any sudden or suspicious moves on the track before the collision with Ward Jr. 

Investigators say they called in multiple accident reconstructionists -- who evaluated the speed of Tony's car, reaction times, distance and other relevant factors. 

NASCAR star Tony Stewart will NOT face criminal charges in the death of Kevin Ward Jr. ... who was struck and killed by Stewart during a race last month ... TMZ Sports has learned. 

We've learned ... that a grand jury just decided not to charge Stewart. 

On the night of his death, Ward Jr. had gotten out of his car and was aggressively marching toward Stewart's car -- when he was struck.

There was talk that Stewart was angry with Ward Jr. and may have intentionally veered his car into him -- but apparently the grand jury feels it was nothing more than a freak accident. 

Investigators had said they had "enhanced video" of the incident -- but clearly the footage did not persuade the grand jury that Stewart did anything illegal.