Finally got some more info on this. Ken Mahnken was in the back seat of a car that was being driven by his sister's Laura's then husband, Mike.
They were on the 610 Loop in Houston near the Woodway exit, somewhere around 8 p.m. on the 29th of September, 1979. He died on the the 5th of October and was buried on the 8th, the day after his 21st birthday.
At the time of the shooting, they were all going to some club or something, where they were supposed to meet Ken's other sister, Eileen and a friend.
Ken was shot in the head. I just sent a note to get more info on this.