The first warning sign came before the plane had even taken off.

Chad Cameron Camp had his choice of seats on the half-empty American Airlines flight from Dallas to Portland, Ore. But Camp, 26, curiously chose a middle seat — right next to an unaccompanied 13-year-old girl, the FBI said in a statement.

Flight attendants offered to move Camp to another seat where he would have more room, but he declined.

“No, I’m fine,” he said, according to a criminal complaint obtained by The Washington Post..

When a flight attendant returned for drink service a half hour later, she saw Camp’s hand on the teenager’s crotch, according to the complaint.

She also saw “a single tear coming down the victim’s cheek.”

Flight attendants separated Camp and the teenager for the rest of the voyage. And as soon as the plane landed, the unaccompanied minor was rushed off the plane.

When Camp exited the plane, he was arrested and charged with abusive sexual contact, according to the FBI.

The teenager’s attorney, however, says that the airline failed to protect its vulnerable passenger.

“This was 30 minutes of hell for this young lady,” said Brent Goodfellow, a lawyer representing the girl, who scoffed at the idea that his client had been saved by a heroic flight attendant.

“If I have my tray table down or my seat back two inches during the improper time, those guys are going to be on me immediately,” Goodfellow told The Post. “This girl got abused for 30 minutes and no one was to be found.”

Not only did the airline fail to protect her, he said, but American Airlines also charged the girl’s family extra to let her travel alone.

“The family paid $300 extra and this is what they get?” he said, adding that his clients are “absolutely going to sue the airline.”

American Airlines said it takes “these matters very seriously” and is fully cooperating with law enforcement.

“American cares deeply about our young passengers and is committed to providing a safe and pleasant travel experience for them,” the airline said in a statement to CNN.

As Goodfellow points out, however, this is not the only recent case of a child allegedly being sexually abused aboard an American Airlines flight.

In July 2015, a Pakistani doctor was similarly accused of groping an unaccompanied girl aboard an AA flight from New York to Chicago. Mohammad Asif Chaudhry, 57, was visiting relatives in the United States last summer when he allegedly moved from his assigned seat to sit next to the girl, who was between 12 and 16 years of age.

The girl woke up from a nap to find the doctor inappropriately touching her, according to a lawsuit filed in federal court against Chaudhry and American Airlines.

“I can’t move cause the seat belt sign is on and I want to get away,” the girl said in desperate texts to her mother, the Associated Press reported. “Mommy, I’m scared.”

Chaudhry said that the contact was inadvertent, however. He was acquitted by a federal judge in November. The lawsuit against him and American Airlines continues in an Iowa court.

After the incident, American Airlines issued a nearly identical statement to the one last week.

“It’s a mockery,” Goodfellow said of the boilerplate statements. “It’s like a slap in the face.”

He said the airline’s reaction was also identical: Move the accused man to the back, move the girl to the front and call the FBI, which handles incidents aboard flights.

Wednesday’s incident allegedly began brewing the moment Camp boarded the airplane.

The girl was flying home to the Portland area after visiting her father in San Antonio. She boarded the flight first.

Camp was wearing headphones and talking to himself when he sat down in his assigned seat, 21-B, right next to the girl.

“The first word out of his mouth was ‘F—,'” the attorney said.

If that wasn’t enough of a warning, Goodfellow said, then flight attendants should have realized there was a problem when Camp declined to move to an empty row — or at least to the aisle seat.

“That’s a big red flag,” the attorney said. “Anybody who has ever flown more than a couple of hours and knows that if there is any empty seat that is not the middle seat, of course they are going to move.”

Camp mumbled and cursed to himself before attempting to make small talk, according to the complaint. He began leaning toward her, forcing her to lean away.

Camp then allegedly began brushing up against her upper arm and shoulder while ostensibly turning the pages of a magazine.

“Each time he turned the page he used his elbow to brush up against the victim’s shoulder and upper arm area. The victim attempted to move away from Camp’s physical contact, and each time she withdrew, he would laugh,” the complaint states. “After finishing with the magazine, Camp leaned toward the victim to place the magazine in the seat pocket in front of the victim. Camp instead dropped the magazine on the victim’s shoes.”

Later, while pretending to look out the window, Camp leaned across the girl again, putting his face just inches from hers, the complaint continues.

When the girl declined Camp’s offer to share his headphones, he allegedly threw them in her lap.

Camp then repeatedly placed his hand on the girl’s knee and upper thigh, according to the complaint.

“She had to push his hand off of her and cross her legs,” Goodfellow told The Post.

“What are you doing?” she asked him. But Camp just laughed and did it again, according to the complaint.

“She was frightened and trapped,” the complaint says.

“She was touched over 15 times,” Goodfellow said.

The incident only stopped when a female flight attendant came by to serve drinks and spotted Camp’s hand on the girl’s crotch, according to the complaint.

That’s also when the flight attendant noticed that the girl was crying.

When confronted, Camp denied doing anything wrong, according to the complaint. And when the flight attendant told him to move, he initially tried to slide over just one seat, to the aisle, according to Goodfellow. Instead, the flight attendant moved Camp to the back of the plane and put the girl near the front.

As soon as the plane reached its gate in Portland, the girl was ushered off so that she could meet her stepfather and give a statement to authorities.

When Camp exited the plane, he was detained by Port of Portland officials and then arrested by FBI agents, according to a statement from the bureau.

Camp later pleaded not guilty, the Oregonian reported.

According to the complaint, another passenger on the flight had seen Camp consume “four mixed alcoholic drinks” in an hour while sitting at the airport bar, although he did not appear drunk at the time.

Even after Camp’s arrest, the girl’s terrifying ordeal wasn’t over.

Before he could be taken to Multnomah County jail, the girl happened to walk past Camp, then detained, on the way to the airport restroom — an experience Goodfellow described as “absolutely traumatizing.”

“She could see Camp [in the airport terminal] and returned to her stepfather frightened and crying,” according to the complaint.

The attorney accused American Airlines of “extorting” separated families by charging them extra to let unaccompanied minors travel without providing any meaningful protection.

The girl’s father paid an additional $300 round-trip for her to visit him.

In August 2014, American Airlines began requiring parents to pay $150 extra each way for unaccompanied minors ages 12-14 to fly. Previously, only unaccompanied minors ages 4-11 were required to use the service.

“This age range not only ensures the safest possible travel for our youngest customers, it’s consistent with US Airways policy before the close of our merger,” American told its employees, according to the Dallas Morning News.

“Our unaccompanied minor service is to ensure your child is boarded onto the aircraft, introduced to the flight attendant, chaperoned during connections and released to the appropriate person at their destination,” the airline’s website states.

“To me, what this is, is American looking for some extra dollars after they’ve got their baggage fees and all the other things they are now hitting us with,” Goodfellow told The Post. “If I’m an executive at American Airlines, I’m thinking: Almost half of our marriages in the United States end in divorce, and there are people moving out of state all the time, and there isn’t a dollar figure out there that people aren’t going to pay to see their loved ones.”

“Parents are assuming they are going to be getting bodyguard service” for $300, but they are not, he added.

Camp never should have been given a seat next to the girl in the first place, Goodfellow argued. He cited some airline policies barring men from sitting next to unaccompanied minors.

Those policies have come under attack, however, from men who claim that they are sexist.

In 2012, an Australian firefighter said he felt like a “pedophile” when a Virgin Australia flight attendant asked him to move from his seat next to two unaccompanied boys, according to CNN. After a public uproar, Virgin said it was reviewing its policy.

“Basically, he felt like a pedophile precisely because he was treated like one,” Joshua Gans wrote in Forbes shortly after the incident.

Other airlines, including British Airways, Qantas and Air New Zealand have also been criticized for similar policies.

Boris Johnson, ex-mayor of London, wrote an op-ed in 2006 about being asked to move on a British Airways flight before he revealed to the flight attendant that the kids were actually his.

British Airways changed its policy in 2010 after being successfully sued by Luxembourgian businessman Mirko Fischer, who complained that he had been treated like a “child molester” by the airline, according to the Telegraph.

Allegations of abuse against unaccompanied minors are relatively rare. In 2013, an off-duty Delta pilot was accused of groping an unaccompanied girl on a flight from Detroit to Salt Lake City.

Such cases date back to at least 1990, when a Michigan man was arrested for allegedly fondling a 9-year-old girl who was traveling with her 7-year-old sister aboard a Northwest Airlines flight.

“Airlines and flight attendants say it’s the first such incident they know of and doubt it could happen except under unusual circumstances,” the Seattle Times reported, noting that the alleged incident happened during an overnight flight when the lights were dimmed.

At the same time, however, the newspaper said the incident had “raised a new concern: youngsters being sexually molested en route.”

Goodfellow said airlines don’t necessarily need to revert to bans on men sitting next to unaccompanied minors.

He said there are simple, nondiscriminatory steps an airline can take, such as sitting unaccompanied minors in aisle seats in front or rear rows, where they are closest to and most easily seen by flight attendants.

Whenever possible, unaccompanied minors should also have an empty seat next to them for protection, he said.

On Thursday, Camp appeared briefly in U.S. District Court in Portland. His attorney entered a not guilty plea on his behalf, the Oregonian reported. The attorney could not be reached for comment late Sunday night.

No matter the outcome of Camp’s case, Goodfellow said the incident had permanently affected the 13-year-old girl.

“She doesn’t want to be on an airplane ever again,” he said. “This is going to affect the rest of her life.”

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