WASHINGTON -- Metro has circulated an internal memo asking employees to be on the lookout for two men seen videotaping the L'Enfant Plaza Metro station.

An internal memo says the individuals were "attempting to videotape inconspicuously, by holding the camera at their side, between their chest and waist." Metro was alerted by a rider who took a picture of the men last week while they were sitting on the train.

The alert comes just after the recent arrest of a Northern Virginia man in a sting where he videotaped two Metro facilities for what we thought was going to be an attack on the system.

Metro says in a statement Thursday that the alert was part of "routine follow-up," not intended to be a public notice and not meant to alarm anyone.

The memo was first obtained by WUSA.