WASHINGTON (Reuters) -U.S. Transportation Secretary Pete Buttigieg said the department was investigating why Delta Air Lines passengers remained on board a plane at the Las Vegas airport that sat on the tarmac in extreme heat on Monday, citing reports that the plane sat there for four hours. "I want to know how it was possible for passengers to be left in triple-digit heat onboard an aircraft for that long," Buttigieg told Reuters on Thursday, calling the issue "infuriating" and "shocking." During tarmac delays, airlines must provide comfortable cabin temperatures and begin moving the plane within three hours of a delayed domestic flight to a location where passengers can exit.