This story was updated at 4:35 p.m.
Driver fatigue and drug use led to the nine-vehicle crash on Interstate 75 in Chattanooga in 2015 that left six people dead, according to the National Transportation Safety Board.
In its release of the probable cause findings for the June 2015 crash, the NTSB also cited a failure in the employee screening process to determine that driver Ben Brewer, 40, of London, Kentucky, had been fired from a previous trucking job two years earlier because of illegal drug use, and he had crashed seven times in the three years before the crash in Chattanooga.
A hair test administered under an unrelated court order less than three months before the crash had also turned out positive for meth.
Brewer was using methamphetamine when he crashed into stopped traffic on I-75 north near Exit 11. He'd also gone for about 40 hours without any significant rest, according to investigators, who testified during a hearing today in Washington, D.C.
"The driver in this crash should not have been behind the wheel of a large truck," said NTSB Chairman Christopher A. Hart. "As long as human beings drive trucks, they must be rested and unimpaired."
Investigators said Brewer did not slow down from a speed of about 80 mph despite ample warnings that he was approaching a construction zone with a 55-mph limit on Interstate 75 near Chattanooga. There was no indication of heavy braking or that he tried to take evasive action before causing the crash, which involved seven vehicles occupied by 18 people.
Brewer has been charged with six counts of vehicular homicide and is custody at the Hamilton County Jail. He is being held on $500,000 bond while awaiting trial in Chattanooga.
The NTSB recommended in its report Tuesday that states make it easier for trucking companies to check prospective drivers' safety and drug records and that more should be done to address the disproportionate number of fatal truck crashes in work zones.
While large trucks make up about 8 percent of all highway miles driven, they accounted for about 30 percent of fatal crashes in work zones in 2014, according to the NTSB.
Read an in-depth report into the crash here: timesfreepress.com/trucking
The Associated Press contributed to this story.