US Homeland Security warns of threat to Washington D.C. metro system

NBC4 obtained an internal memo from Homeland Security about the concern. Someone traveled to Turkey last July to obtain a U.S. visa to come to the States to blow up an unspecified Metro station, the memo said.

"The information that we have has a low level of credibility," said David Webb, of the Metro Transit Police. "There is not a lot of information that suggests a time or place or that this person is even capable of conducting such a crime."

Metro said commuters should always be vigilant and report any suspicious activity. Since December, 20 more Metro security officers have been added to patrol the 1,500-square-mile system investigating even low level threats like this one.