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Frontier Airlines is facing criticism from a top police officer organization and Republican Rep. Doug Collins after they were accused of prohibiting a federal air marshal from boarding a flight in Atlanta due to his face covering, which featured an American flag.

The National Association of Police Organizations first flagged the incident at Hartsfield-Jackson Atlanta International Airport on Aug. 23, when the air marshal "was told to remove his American Flag face mask because other passengers would find it offensive."

The airline has since called the incident a misunderstanding, saying that the airline has a policy banning certain styles of masks, but not the image of a flag. The federal air marshal (FAM) was wearing a gaiter-style mask.

"The matter was a clear misunderstanding on the part of both parties," Frontier said in a statement to Fox News Tuesday. "The gate agent mistakenly believed that the passenger’s gaiter style mask was not acceptable within our mask policy and the passenger grossly misinterpreted the reasoning behind her objection to his mask."

NAPO, last week in a letter to president and CEO of Frontier Barry Biffle, said that while attempting to board the plane, air marshal was "stopped by the Gate Agent" and "advised that he should wear another mask and was handed a disposable one."

"The FAM politely put the disposable mask on and then covered that mask with this American Flag mask," NAPO wrote. "The Gate Agent then told him to remove the American Flag mask or he would not be allowed to board the plane as it would create issues with the other passengers."

NAPO went on to claim that "several passengers with Black Lives Matter masks boarded the plane."

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"The fact that a Frontier Airlines Gate Agent told the FAM to remove his mask because the image of an American Flag would inflame tensions with other passengers is indefensible," NAPO wrote, adding that it was "insulting that an American Flag face mask would be deemed so controversial."

"Wearing an American Flag face mask is not a political statement; it is proudly showing support for our country," NAPO added. "The Gate Agent made it political, particularly given the other masks that were allowed without issue on the plane."

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Meanwhile, Rep. Doug Collins, R-Ga., who is running for U.S. Senate, also weighed in, penning a similar letter to Biffle.

Collins acknowledged that Frontier has "looked into this matter and determined the gate agent in this case misunderstood" the mask policy, but said that "logic simply does not follow when coupled with the Air Marshal’s personal experience."

"Assuming your mask policy does not prohibit passengers from wearing two masks, it is clear that your gate agent did not simply misinterpret the policy," Collins wrote.

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Collins claimed that the gate agent "chose to prohibit this passenger from boarding the flight simply because they did not like the content displayed" on the air marshal's mask.

"This infringement of a law enforcement officer's right to free speech was clearly a decision made by one of your gate agents based on their own politics," Collins wrote, calling the incident "disappointing."

Collins went on to request that the airline "immediately make clear that its policies permit the display of the American Flag on clothing and face masks and that no passenger should be denied service because they are honoring the American Flag on their clothing."

"To be clear, no airline should infringe upon a passenger's First Amendment rights, much less those of a law enforcement officer traveling for the very purpose of protecting your passengers, because other passengers might find their patriotic clothing 'offensive,'" Collins wrote.

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Collins also demanded a "full personnel investigation" into the incident and "take any adverse action against the gate the gate agent involved as your company deems necessary to protect the free speech of its passengers from any future politically driven actions of this particular case agent."

He added: "I hope Frontier Airlines will work to respect the freedoms we cherish and the bedrock principles that make our nation the best country on earth."

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Meanwhile, a Frontier spokesperson told Fox News that: "We welcome, respect and revere our nation’s flag and the notion that we object to the flag emblem on our aircraft is simply ridiculous."