A Southwest flight appeared to take off from a closed runway that had a vehicle on it without being cleared by air traffic control

• A Southwest Airlines flight took off from a closed runway despite air traffic control warnings.
• The pilots didn't communicate with the tower or other planes, an ATC recording suggests.

A Southwest Airlines flight appeared to take off from a closed runway that had a vehicle on it, despite air traffic control trying to warn the pilots.

The National Transportation Safety Board is investigating the incident.

Flight 4805 departed Portland, Maine for Baltimore at 5:42 a.m. on Tuesday, per Flightradar24. However, a NOTAM — a notice to flight crews — said that Runway 29 was closed for takeoff until 5:45 a.m.

In an audio recording published by LiveATC.net, the controller can be heard cautioning the Southwest pilots as the plane departs.

"There is a vehicle on the runway. The runway is closed," he said. Another person added: "I tried warning him."

Six minutes later, ground operations asked the controller: "Did you ever get a hold of that Southwest plane once he's airborne or is he still kind of MIA in that regard?"

"We never talked to the aircraft," he replied.

The Portland ATC tower doesn't officially open until 5:45 a.m. Instead, pilots are supposed to communicate on the common traffic advisory frequency (CTAF) so other pilots know what's going on.

However, it appears that the Southwest pilots didn't talk on this radio frequency either.

Per the ATC recording, the controller told ground operations: "You were on the runway and had to exit, and he wasn't even on CTAF."

In a statement, Southwest said it is engaged with the NTSB and Federal Aviation Administration to understand what happened. "After departure, the aircraft continued safely to its destination," it added.

Portland International Jetport did not immediately comment, noting the active NTSB investigation.

The NTSB said in a statement that a preliminary report will be available in 30 days.

The incident is one of several to involve a Southwest plane in recent months. On June 18, Flight 4069 was just 500 feet above the ground while still nine miles away from the airport — prompting a low-altitude alert at air traffic control.

Also this month, a Southwest pilot accidentally sent his plane into a dive before it avoided crashing into the ocean near Hawaii.

And in May, a Southwest 737 Max experienced a rare but serious stability issue known as a Dutch roll. The jet has since been out of service for over a month while the NTSB investigates.