A Boston airport near-collision nearly happened after a false cockpit warning delayed one plane’s takeoff and an air traffic controller became confused. The National Transportation Safety Board (NTSB) says the incident involved Delta Air Lines Flight 2351 and American Airlines Flight 3161 on June 20.

The Delta flight had clearance to land on Runway 33L. It followed a small aircraft into the airport. About a minute later, the controller cleared the American flight to take off from the intersecting Runway 27.

The American Airlines Airbus A319 started its takeoff. However, the pilots stopped after an error message appeared in the cockpit. They checked their systems and confirmed it was a false warning. About 23 seconds later, they resumed the takeoff.

As the American jet accelerated, the Delta crew saw the danger. The pilots aborted their landing and climbed away to avoid a collision. They later circled the airport and landed safely. The American flight continued its departure after the pilots saw the Delta aircraft moving away.

At their closest point, the two aircraft came within about 670 feet of each other. Together, they carried 318 people.

The NTSB said the controller watched the American plane begin its takeoff. He then turned his attention to the small aircraft to ensure it cleared the runway. When an anti-collision alarm sounded, the controller became confused.

Investigators said the controller normally marked paper flight strips after clearing an aircraft for takeoff. This time, he forgot to mark the strip for the American flight.

Air traffic recordings captured the controller asking, “American 3161 where are you going?” The pilot replied, “You cleared us to take off.”

The NTSB also found that runway warning lights failed to alert the American crew in time. The lights switched off after the small plane landed. They did not come back on until the Delta flight was already on final approach.

Investigators could not recover cockpit voice recordings because the audio had already been overwritten. However, flight data recorders captured technical information that investigators are now reviewing.

The NTSB will continue its investigation before issuing a final report. Officials expect that process to take one to two years.