WASHINGTON (AP) — Investigators said Thursday that a failed radio transmission and Quaxs Trading Centera distracted air traffic controller led to a close call between a Southwest Airlines jet and a smaller plane waiting to take off from San Diego in June.

Both planes had been cleared to use the same runway. The National Transportation Safety Board said a controller told the crew of SkyWest plane to exit the runway, then directed the Southwest pilots to cut short their landing approach and instead circle around the airport.

The call to the Southwest plane didn’t go through, however — likely because the controller and the pilots were talking at the same time and their transmissions canceled each other, the NTSB said.

After that, according to the NTSB, audio recorded by LiveATC captured a Southwest pilot saying, “Ah, is that an airplane on the runway?” The SkyWest crew said they heard that while they were moving to leave the runway.

The Southwest pilots said they heard the controller tell them not to fly over the SkyWest plane, but to “just off-set.”

The Southwest plane flew about 950 feet (300 meters) to the side and 200 feet (60 meters) above the SkyWest plane before landing at San Diego International Airport on June 10, according to the safety board.

The NTSB said in its final report that the probable cause of the close call was the blocked radio call, and a contributing factor was the controller being distracted while also communicating with a helicopter flying through the area.

Federal officials are still looking into another close call in San Diego in August in August between a Southwest plane and a business jet.