Updated

The National Transportation Safety Board announced it is launching an investigation into the 133-car pileup that left six people dead and dozens injured in Fort Worth, Texas Thursday. 

The investigation will look into the snow and ice treatment procedures for the roads, the board said. 

The agency responsible for treating Interstate 35W, the North Tarrant Express, said last week that the roads were treated throughout the week. 

"NTE & NTE35W maintenance crews started pre-treating the corridors on Tuesday morning in anticipation of inclement weather, and have been treating continuously as they monitor the roadway," North Tarrant Express said in a statement to WFAA, the ABC affiliated TV station in Dallas, last week. 

"We will continue treating the highways through the weekend and into next week, as long as the storm is active. Our crews treat the entire corridor, managed lanes, general purpose lanes, frontage roads and ramps."

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The North Texas Toll Authority said that Texas roads are treated with brine, "a heavily salted liquid solution that dries and helps treat roads prior to inclement winter weather."

Texas lawmakers are calling for an investigation into when and how the roads were treated. 

"I have spoken to state legislators from the region, and they expressed concerns that this roadway may not have been sufficiently pre-treated for icy weather prior to the event," Texas state Rep. Terry Canales, the Chairman of the House Transportation Committee, told Nexstar.

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"If this is true, regardless if it is the responsibility of a private entity to treat the North Tarrant Express, it is wholly unacceptable, and I’m calling on the Texas Department of Transportation and the Texas Department of Public Safety to conduct a complete investigation into the circumstances surrounding Thursday morning’s traffic disaster."

[WARNING: THIS VIDEO MAY BE DISTURBING TO SOME VIEWERS]

The 133-car pileup started around 6:15 a.m. Thursday morning as cars lost control on a thin sheet of ice that covered the road. A video posted by Dallas Texas TV, a local social media company,  showed an 18-wheeler unable to stop as it crashes into multiple cars. 

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"The roadway was so treacherous from the ice that several of the first responders were falling on the scene," Matt Zavadsky, a spokesman for MedStar Mobile Helathcare - which sent multipple ambulances to the crash -- told the Associated Press last week. 

Smaller pileups happened throughout Texas last week, and drivers should stay on high alert now as a winter storm barrels through Texas.