UPDATE, 3:28 p.m.

Metro interim general manager Richard Sarles announced today that the agency will begin performing random checks of passengers' bags and packages in the coming days.

Metro officials have said the screenings will take "only minutes," are intended to be "non-intrusive," and could go on indefinitely. Transit officers will use ionization technology and K-9 units to try to detect explosives inside Metro stations. Anyone who's pulled aside for a screeing and refuses to submit their bag or package will be prohibited from bringing it onto Metro. Metro says the bags will "generally not be opened" unless officers believe there's a need for further inspections.

The agency has played with the idea of random inspections in the past but hasn't instituted any until now.

"They could start as soon as this afternoon," said Metro Transit Chief Michael Taborn. "It's another tool in our toolbox."

Taborn said the screenings will be completely random and they'll be done inside stations before passengers have swiped their farecards. Riders who have carry-on items may be asked to come over to a table where transit officers and perhaps agents with the federal Transportation Security Administration are stationed. "We'll be working in partnership with TSA and probably many of their screeners," Taborn said.

At that point, a cloth-like swab will be run over the bag and placed into the ionization machine that detects explosives, and an explosives dog may also inspect the bag.

Taborn said that transit officers will not perform an inspection without permission and that any rider can decline to have his or her bag inspected and leave the station. Asked if an agent would likely follow up with that rider, Taborn said, "There's always a possibility."

Taborn declined to say how frequently they would screen passengers, saying that would undermine the program, but he said the agency would be considerate of how busy stations are and would avoid creating bottlenecks. The agency may choose to have screening tables set up at bus stations as well, so Metrobus riders may be asked to have their bags screened as well.

The Riders' Advisory Council was not consulted on the decision, but Metro spokeswoman Lisa Farbstein said riders in New York have been receptive to a similar screening.

Taborn said the decision to implement screenings now had nothing to do with a recent Metro bomb plot or with funding. He said the agency already has the equipment and manpower in place and that a series of grants from TSA will mean the initiative "won't cost anything."

Explaining the rationale behind the screenings, he said, "If you can throw the bad person off, if we can alter that, then we’re successful.... There's no better time than now to add additional security measures to our system."