Metro Transit Police are urging officers to be vigilant to threats from jihadist terrorism.
However, the Washington Metropolitan Area Transit Authority emphasizes that there are no current threats known against the subway and transit system.
Metro on Friday released a redacted copy of a threat awareness bulletin dated Aug. 21. In it, Chief Ronald Pavlik says that recent events overseas, including recent slayings by Islamic State militants, show the need for the force to stay alert.
The memo also says the coming anniversary of the Sept. 11, 2001, requires increased vigilance.
A spokeswoman for Metro says there is no threat currently against the system.