A man from the US state of Virginia was arrested on Wednesday for allegedly plotting to help Al-Qaeda bomb metro stations in Washington DC.
Farooque Ahmed, 34, was born in Pakistan but is a naturalised US citizen.
The suspect allegedly conducted surveillance of a metro station on the outskirts of the American capital on four occasions
Ahmed was taken into custody early on Wednesday morning after a federal grand jury indicted him on three terrorism-related charges.
If convicted, he faces up to 50 years in jail.
FBI investigators say they had been monitoring the suspect months before the alleged plot began and insist that the general public were never in any danger.