A “mass casualty incident” is unfolding on Interstate 35W just north of downtown Fort Worth, where dozens of vehicles collided and left drivers trapped, firefighters say. Mike Drivdahl with the Fort Worth Fire Department said firefighters are going car-to-car, looking for people in need of medical care. Preliminary details are unfolding and it is not yet known how many people have been hurt. Drivdahl said it will “take a lot of time” to clear the damage. “If you don’t have to get out, please don’t get out,” Drivdahl said in a live phone interview on NBC 5. “We have a lot of emergency crews doing a lot of work. The last thing we need is people who don’t need to be out getting into any type of accidents.” Crews are dealing with slippery conditions and using anything they can — sand, salt and kitty litter — to melt the ice as they triage patients injured in the crash, Drivdahl said.