A Pakistani-American was on Wednesday arrested for plotting attacks on subway stations in and around Washington with people he believed were tied to Al-Qaeda, US officials said.
Farooque Ahmed, 34, who lived in the Virginia suburbs of Washington, had been allegedly casing and photographing subway stations since April to plan simultaneous bomb attacks.
"Farooque Ahmed is accused of plotting with individuals he believed were terrorists to bomb our transit system, but a coordinated law enforcement and intelligence effort was able to thwart his plans," David Kris, assistant attorney general for national security told reporters.
The White House said swiftly the US public was never in any danger from the alleged plot, and that President Barack Obama had been aware of it before Ahmed's arrest.
White House spokesman Robert Gibbs said the Justice Department, the FBI and national security officials had been on "top of this case from the beginning."
"At no point was the public in any danger," he added.
Ahmed, a naturalised US citizen who was born in Pakistan, was due to appear in a court in Virginia today.
He faces charges of attempting to provide material support to a terrorist organisation, collecting information to assist in planning a terrorist attack, and attempting to provide help to carry out multiple bombings and cause mass casualties in Washington.
If convicted, he could be jailed for up to 50 years.
"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," said US Attorney Neil MacBride.