The crash happened Nov. 15 during a parade honoring veterans in Midland, the San Antonio Express-News reported.
Dale Wayne Hayden, 50, drove a float carrying 12 veterans and their wives through an train-crossing intersection just after the crossing's warning system had been activated.
The float was then struck by a freight train traveling 63 mph.
Four veterans -- Army Sgt. Joshua Michael, 34; Army Sgt. William Lubbers, 43; Marine Chief Warrant Officer Gary Stouffer, 37; and Army Sgt. Maj. Lawrence Boivin, 47 -- were killed in the collision and 16 others were injured
Wednesday, a Midland grand jury declined to indict Hayden, but called for further investigation into the incident.
"The grand jury has not concluded its review of the incident," Midland County District Clerk Ross Bush said, "but has returned a no-bill in the matter of the driver of the truck, Dale Wayne Hayden."
Assistant District Attorney Eric Kalenak said the grand jury will issue a report at some point in the future that will include recommendations on "how these kinds of things could be prevented."
"The grand jury discussed it extensively," Kalenak said. "It's safe to say that this was a very traumatic thing for this community, and there were some aspects that they wanted to look into."