MSNBC is reporting that the Qatari is a diplomat.

United Flight 663, a Boeing 757 with 157 passengers and six crew members aboard, landed safely at Denver International Airport, airline and airport officials said.
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Sources told NBC News that the passenger who was subdued was a Qatari diplomat.

And that could potentially explain how the guy got through security (note that 24 hour rule in effect on confirming identity of man involved).