The US Justice Department says a suspect has been arrested for plotting a subway attack in the US capital, Washington DC.

The accused, Farooque Ahmed, is alleged to have attempted to assist others, who he believed to be terrorists, to plan the multiple bombings.

Officials say the FBI was closely monitoring Ahmed's activities during the investigation and say there was no threat to Metro rail or the public. 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."