The plot has thickened in the highly unusual mishap, which saw Delta Airlines Flight 89 dump thousands of gallons of volatile jet fuel on unsuspecting students and staff in five elementary schools and one high school outside Los Angeles on Tuesday.

An audio recording of communications between the LAX airport control tower and the pilots of the Delta flight reveals that air traffic controllers asked the crew twice if they needed to get rid of any excess fuel before making an emergency landing due to an issue with the craft’s right engine.

At one point in the exchange – surprisingly calm given the situation – the controller can be heard asking the pilot if he was going back to the airport “immediately” or would need to “hold and burn fuel.”

At first, the pilot simply dodges the question, saying that the situation “is not critical” and that he was turning the aircraft back to the airport. As the controller presses further, asking one more time: “OK, so you don’t need to hold to dump fuel or anything like that?” – the pilot comes up with a rather definitive ‘no.’ “NEGATIVE,” he says.

The audio was obtained through LiveATC.ne, a site that provides a live feed of Air Traffic Control communications around the world, and released in full by the Los Angeles Times.

It remains a mystery what transpired in the next twenty minutes that forced the pilot to make an about-face and jettison the fuel over heavily populated areas – home to multiple elementary schools – with the Federal Aviation Administration (FAA) now saying that the Delta Flight 89 failed to notify controllers of the move.

“A review of yesterday’s air traffic control communications shows the Delta Flight 89 crew did not tell air traffic control that they needed to dump fuel,” the US aviation watchdog said in a statement on Wednesday, noting that the drop-off “did not occur at an optimal altitude that would have allowed the fuel to atomize properly.”

While FAA guidelines do not expressly prohibit dropping fuel at such a low altitude – about 2,300 feet at the time of the incident – air controllers typically direct crews to dump it over water or unpopulated terrain. It’s standard practice for pilots to drop fuel at an altitude of over 10,000 feet, which ensures the substance evaporates on its way to the ground.

That the move was not per se illegal, however, will likely come as small comfort to dozens of schoolchildren caught off guard and drenched in the toxic downpour. The incident sowed panic on the ground, and led to about 60 people being treated for skin irritation and breathing issues. Some had to be decontaminated with soap and water.

In a statement on Wednesday, Delta, which argued that fuel dump “was required as part of normal procedure to reach a safe landing weight,” said it had sent “13 cleaning crews” to assist local authorities in cleansing all contaminated areas around the schools.

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