(CNN) — A source familiar with the near-collision of two airliners on a John F. Kennedy International Airport runway tells CNN that special flashing lights which warn pilots against taxiing across a runway were in fact working.

The new development further raises the possibility that human error contributed to the Friday evening incident.

The incident involved an American Airlines 777 that improperly crossed an active runway at the busy airport. “S**t!” an air traffic controller called out over the radio, urgently directing a Delta Air Lines 737 to stop its takeoff from the same runway where the American plane rolled across its direct path. The Delta plane stopped within 1,000 feet of the American jet.

On Wednesday, the source told CNN that following the incident, airport staff “went out immediately” to confirm Runway Status Lights were working properly.

JFK is one of 20 airports in the United States equipped with the fully automated Federal Aviation Administration system that warns pilots “when it is unsafe to enter, cross, or takeoff from a runway.”

On Tuesday, the National Transportation Safety Board told CNN that “interviews are ongoing” as part of its investigation into the incident. The FAA is also investigating.

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