At least six people have died and dozens more were injured in a massive pileup involving at least 133 vehicles on the North Texas Express toll lanes in Fort Worth early Thursday morning that occurred after overnight sleet made for treacherous driving conditions.

All southbound lanes of Interstate 35W north of downtown, including the interior toll lanes where the crashes took place, remained closed through the day and overnight as dozens of firefighters and other first responders continued to work in bitterly cold temperatures to clear the damaged vehicles from the highway.

The interstate remains closed as of 7 a.m. Friday as crews continue to clear the scene of debris.

Dubbed a “mass casualty incident” by firefighters earlier in the day, officials said during a news conference Thursday afternoon that at least 65 people are receiving treatment at area hospitals including 36 who were transported from the scene by ambulance.

MedStar says 13 ambulances were used at the scene.

Medical City Hospital, Cooks Children’s, Texas Health Resources and JPS Hospital all reported taking in patients.

6 Killed, Dozens Hurt as 130+ Vehicles Collide on ‘Sheets of Ice’ in Massive Fort Worth Pileup