CNN  —  Airport employees in Portland, Maine, were forced to clear out of the way when a Southwest Airlines jet on the morning of June 25 took off from a closed runway, according to air traffic control recordings. Controllers and pilots warned the crew of Southwest 4805 several times that the runway was closed, the recordings made by LiveATC.net show. The incident is now under investigation by the National Transportation Safety Board and Federal Aviation Administration, and Southwest says it is engaged with both regulators. For the last several months the runway has been closed overnight for a taxiway construction project, and it reopens each morning at 5:45 a.m., according to the Portland International Jetport. A controller said on the recordings that the tower is also closed overnight. “There’s an airport vehicle,” one voice warned over the radio. Said another:  “The Southwest pulling on to 29, just so you know there is a vehicle on the runway and it is still closed.” But the Southwest pilots apparently never heard the warnings.  They were recorded communicating on a different radio frequency with regional air traffic controllers near Boston. The Boston controller later told Southwest they departed three minutes before the runway reopened. Boston controller: “That runway was closed.” Southwest pilot: “I thought it opened at :45.  There was no markings or anywhere.” Controller: “It did open at :45, but you departed at :42.” Pilot: “That’s not what we’re showing, but ok.” The FAA told CNN the runway was indeed closed at the time, and that the “airport vehicle exited the runway before the plane began its takeoff roll.” It is unclear how close the plane came to colliding with the runway crew.  The tower controller told the workers over the radio that he knew “you were on the runway and had to exit” because of the takeoff. Southwest said the flight “continued safely to its destination,” and referred questions to the NTSB, which said it is investigating and would make public a preliminary report on the incident next month. This is the latest in a series of safety incidents involving Southwest airplanes. Investigators are also probing recent Southwest flights that came dangerously close to crashing near Oklahoma City and one of the Hawaiian islands, as well as a rare, unsafe in-flight swaying known as a Dutch roll.