|Thousands of commuters use the Washington DC subway network each day [GALLO/GETTY]
US officials have arrested a Pakistani-American man for plotting a series of bomb attacks on Washington's subway system with people he believed were tied to al-Qaeda.
According to the US justice department, 34-year-old Farooque Ahmed of Ashburn, Virginia was arrested on Wednesday morning after a federal grand jury returned a three-count indictment on Tuesday against the naturalised US citizen who was born in Pakistan.
Federal authorities have been monitoring Ahmed's activities since April 2010 and they have said that the public was never in danger during the investigation.
Earlier this month, both the United States and Britain warned of an increased threat of terror attacks across Europe.
However, a US official speaking on condition of anonymity told the Reuters news agency that there was "no connection between recent reports of the terrorism threats in Europe and this arrest".
He also said that the undercover individuals posing as al-Qaeda operatives "worked on behalf of the government in this matter".
During the six-month undercover investigation, Farooque Ahmed filmed northern Virginia subway stations, suggested using rolling suitcases instead of backpacks to kill as many people as possible and offered to donate money to al-Qaeda's cause overseas.
Robert Gibbs, the White House press secretary, said that President Barack Obama was aware of the investigation before Ahmed's arrest.
David Kris, assistant attorney general for national security, said the case "demonstrates how the government can neutralise such threats before they come to fruition".
But Andrew Ames, a spokesman for the FBI Washington field office, declined to comment on how authorities learned about Ahmed.
Charges against Ahmed
He has been accused of 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 carry out multiple bombings to cause mass casualties.
During a brief court appearance in federal court, Ahmed did not enter a plea and was held in custody.
He told the judge that he could not afford a lawyer.
Neil MacBride, an attorney, said that it was "chilling that a man from Ashburn is accused of casing rail stations with the goal of killing as many Metro (subway) riders as possible through simultaneous bomb attacks".
Ahmed faces up to 50 years in prison if convicted.
The indictment said that between April 2010 and October 2010, Ahmed allegedly conducted surveillance, videotaped, photographed, and drew diagrams of the Arlington Cemetery, Courthouse, Crystal City and Pentagon City Metrorail stations.
He also offered suggestions about where to place explosives, and that between 4pm and 5pm would be the best time for an attack to cause the maximum number of casualties in simultaneous attacks planned for 2011.
A LinkedIn page that was created for Farooque Ahmed identifies him as a network planning engineer with a bachelor's degree in computer science from the City College of New York in 2003.
Recent terror sentences
Since the September 11, 2001 attacks, US authorities have been concerned about the threat of another attack on US soil.
Last week, a Jordanian national was sentenced to 24 years in prison for attempting to blow up a Dallas skyscraper.
Earlier this month, Pakistani-born American Faisal Shahzad was sentenced to life in prison for trying to set off a car bomb in Times Square, New York.