Whatever happens in the air, stays in the air.

That's the likely outcome of an airplane sex case unfolding in Detroit, where the FBI ticketed a man and a woman over the weekend after they were caught in a sex act while in their seats aboard a Delta flight.

According to sources familiar with the case, the couple -- who were reportedly strangers before the flight -- are not expected to be criminally charged for their behavior, but rather will only have to pay fines of up to $800 a piece, maybe more.

As of late Tuesday, the U.S. Attorney's Office said it had received no word from the FBI about seeking any possible charges. The Wayne County Prosecutor's office said it's not involved in the case.

Related: Report: 2 passengers who met on plane accused of sex on flight to Detroit

According to WDIV, the 48-year-old woman was performing oral sex on the 28-year-old man, leading to complaints from nearby passengers.

Former federal prosecutor Peter Henning said he doubts the couple will face any criminal charges. At most, he said, the couple would be cited for lewd and lascivious behavior.

"It’s going to be very hard to find that this is criminal conduct under the federal code because it’s not a threat to the safety of the airline, or other passengers," said Henning, a Wayne State University law professor. "It’s certainly distasteful, but it was not disruptive or interfering with the operation of the plane -- and that’s typically what (airline incident) charges involve ... The embarrassment is probably the biggest punishment they can receive "

According to Detroit Metro Airport spokesperson Erica Donerson, the two passengers at issue were taken into custody at the airport terminal when the plane landed on Sunday. It was coming from Los Angeles. Airport police, who had been alerted about the incident by Delta crew, were there waiting and transported the couple to the airport's safety building.

There, Donerson said, the FBI took over.

"The FBI issued the citations, so they are handling the case," said Donerson, noting she didn't know the exact nature of the citation. "All I know is that (the FBI) responded and took it from there ... We have a police report, so I'm assuming something happened."

Detroit's former FBI Chief Andy Arena said this is a first for him.

"I've never seen anything like this," Arena said. "It's criminal stupidity, felony stupidity."

Arena noted that while he ran the Detroit FBI office, he did see cases where airline passengers were criminally charged for misbehaving on an airplane. But those cases typically involved men who fondled children on a plane, or inappropriately touched women passengers, he said. Then there was the terrorism case that involved the so-called underwear bomber Umar Farouk Abdulmutallab, the self-proclaimed terrorist who tried to blow up a Detroit-bound airliner with explosives hidden in his underwear on Christmas Day 2008.

Arena oversaw that investigation. The bomber's plot was foiled, however, and he's now serving a life sentence.

Arena said while charges are unlikely in the sex case, they're not out of the question. He said it will depend on who was sitting around the couple, whether anyone saw the act, and how disruptive it was, if at all.

"If this happened on the streets of Detroit and the cops rolled up, what would they do?" Arena said, noting the act "technically could" trigger charges. " I don't know what the crime would be. I've never seen anything like it."

FBI spokesman Tim Wiley said the bureau can't comment on "open or ongoing investigations." But he did say the bureau investigates crimes aboard airplanes "once the boarding door closes, regardless of severity."

Delta Airlines declined comment.

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