The Department of Homeland Security warned Metro about an unconfimed threat to the D.C. Metro system on Sunday.

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 where 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.

Officers will be riding on the Red Line today as staging for a counter-terrorism drill that had been previously planned. It’s unrelated to the threat.

(Source: NBC Washington)