A federal air marshal reportedly "flashed a gun in flight," prompting police to remove him from the plane upon arrival at the Twin Cities airport and to handcuff him on the tarmac along with a fellow marshal, authorities said Tuesday.

The apprehensions occurred Monday night after the nonstop United Airlines Flight 3531 from Newark, NJ, landed at the Minneapolis-St. Paul International Airport, according to the federal Transportation Safety Administration.

The marshal was "on official business onboard a flight [and] was mistaken for a passenger by a flight attendant," a TSA statement read. "Protocols for notification of law enforcement presence aboard an aircraft are in place to avoid incidents like this. TSA is working with the airline to determine the specific circumstances in this case."

However, communication between the cockpit and the MSP control tower that was captured by the authoritative website Liveatc.net revealed that it was confirmed onboard fairly quickly that both men were federal air marshals, and one of them "actually showed our flight attendant his gun," one of the pilots reported soon after landing.

"That is completely against SOP [standard operating procedure] for them to show their firearm,” the pilot added. “So that’s the reason we declared an emergency."

Airport spokesman Patrick Hogan said that airport officers were following protocol and it was up to federal and airline officials to explain why the flight attendant failed to realize the armed man was a federal air marshal. Those officials have yet to address how the misidentification occurred, reports the Star-Tribune .