Department of Transportation said Thursday that it was investigating why passengers on a Delta Air Lines flight had been left to swelter for hours in triple-digit temperatures while the plane waited on the tarmac at a Las Vegas airport on Monday. The flight, DL555, to Atlanta, was supposed to take off from Harry Reid International Airport on Monday but instead sat on the tarmac, where the extreme heat sickened passengers, causing some of them to faint, according to reports posted to social media by those on board. “Even at normal temperatures, a tarmac delay is not supposed to go that long, and we have rules about that, which we are actively enforcing right now,” he said