U.S. law enforcement officials received intelligence reports two weeks ago warning of a possible terror attack in Glasgow against "airport infrastructure or aircraft," a senior U.S. law enforcement official tells the Blotter on ABCNews.com.
The intelligence reports also warned that airports and aircraft in the Czech Republic could be the targets of al Qaeda-connected terrorists.
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The warnings were kept secret for operational reasons, according to officials.
In public, the White House and Homeland Security Secretary Michael Chertoff have continued to maintain they know of no specific or credible threats involving the United States, even though the intelligence reports specify U.S. aircraft as possible targets.
A U.S. official told ABCNews.com that the intelligence reports led to the assignment of federal air marshals to flights into and out of both Glasgow and Prague.
Air marshals had been added to flights into and out of Germany late last month based on similar warnings.