A US citizen originally from Pakistan has pleaded guilty to joining what he believed to be an al-Qaeda plot to bomb the Washington DC Metrorail system.
Farooque Ahmed was sentenced to 23 years in prison on Monday immediately after entering his plea.
The 35-year-old admitted to conspiring to provide material support to a designated terrorist organisation.
He also pleaded guilty to collecting information for a terrorist attack on a transit facility.
Ahmed, a resident of the city of Ashburn in Virginia, was arrested in October 2010 following what the FBI said was a six-month sting operation.
Authorities said Ahmed had studied security operations at subway stations last year in northern Virginia, near Washington DC.
Ahmed also took photographs, provided diagrams and made suggestions to US authorities posing as al-Qaeda members on how a bomb could inflict the highest number of casualties, authorities said.
Ahmed was reportedly first lured by email to meetings with authorities on 18 April 2010.
The naturalised US citizen had lived in the country since 1993, FBI agent Charles Dayoub said at the time of Ahmed's arrest.
Investigators said in 2010 that they believed Mr Ahmed had used firearms he purchased in May of 2008 and February of 2009 to train for his goal of travelling to Afghanistan to kill Americans.