The driver of a semi-truck that caused a deadly chain-reaction accident was using methamphetamine and likely hadn't slept, according to the final crash report from the National Transportation Safety Board.

The crash happened on I-75 in Ooltewah, near Chattanooga, on June 25, 2015.

Investigators said Benjamin Brewer slammed into slow-moving traffic in a work zone. The semi was traveling at interstate speeds when it rear-ended one car, causing a chain-reaction crash that eventually involved eight vehicles and killed six people. Among the dead were 31-year-old Tiffany Watts of Morristown and 51-year-old Sandra Anderson of Rutledge.

Brewer was charged with six counts of vehicular homicide and DUI and faces multiple lawsuits.

According to the NTSB report, Brewer's blood tested positive for methamphetamine use, which investigators determined "degraded his driving performance."

The investigation also found that although he had the "opportunity for overnight rest" before the crash, Brewer had likely gone without "sustained rest for 40 hours prior to the accident."

They also cited a failure of Brewers' employer, Cool Runnings Express, for failing to identify risk factors in his pre-employment screening process. The report stated that the company did not have Brewers' entire driving history, including previous accidents.