A man has been arrested over a plot to bomb Metro stations in the US capital.
Farooque Ahmed, 34, of Ashburn in the state of Virginia was conspiring with people he believed were from al-Qaeda, the Department of Justice said.
Mr Ahmed was taken into custody on Wednesday morning. If convicted he faces up to 50 years in prison.
The suspect allegedly conducted surveillance of Metro stations in Arlington, on the outskirts of Washington DC, on four occasions.
Mr Ahmed appeared in a federal court in Alexandria in the state of Virginia, on Wednesday. He did not make a plea and was held in custody.
The suspect told the judge he was unable to afford a lawyer. Prosecutors said they would be using classified information during the case.
Mr Ahmed's arrest is the latest in a line of investigations of US citizens or residents for involvement in alleged domestic terror plots. Last year there were arrests over plans for attacks in Texas, Colorado and Illinois.
More recently Faisal Shahzad was convicted of attempting to bomb Times Square in New York.
"It's chilling that a man from Ashburn is accused of casing rail stations with the goal of killing as many Metro riders as possible through simultaneous bomb attacks," US Attorney Neil MacBride said in a statement.
"Today's arrest highlights the terrorism threat that exists in northern Virginia and our ability to find those seeking to harm US citizens and neutralise them before they can act."
Officials said the public were never in danger during the investigation and that they had been aware of Mr Ahmed since the beginning.
Mr Ahmed, who was born in Pakistan, has been indicted on three counts.
These are attempting to provide material support to a designated terrorist organisation, collecting information to assist in planning a terrorist attack on a transit facility, and attempting to provide material support to help carry out multiple bombings.
The indictment against Mr Ahmed said that on 18 April he drove to a hotel in Dulles, Virginia, to meet someone who he believed was a terrorist courier who had details of locations for future meetings.
Officials said that on 15 May, at a nearby hotel in Herndon, Virginia, Mr Ahmed agreed to carry out surveillance on a hotel in Washington and a Metro station in Arlington, Virginia, to determine security and the busiest times.
Mr Ahmed, according to the indictment, later provided a USB stick with video of the station to a person he believed was from al-Qaeda. On 28 September, Mr Ahmed allegedly handed over another USB drive with more images of stations.
In that meeting, he also allegedly suggested terrorists use rolling suitcases to blow up Metro stations, rather than backpacks.