The FBI learned in January that a man arrested over an alleged Washington DC Metro bomb plot tried to contact extremist groups in an effort to fight US forces overseas, the agency says.
Farooque Ahmed and another unnamed person hoped to fight in Afghanistan or Pakistan early next year, FBI agent Charles Davoub said in an statement.
Mr Davoub said Mr Ahmed may have acquired weapons in 2008 and 2009.
The 34-year-old was arrested on Wednesday in the Washington DC area.
Mr Ahmed is believed to have used firearms he purchased in May of 2008 and February of 2009 to train for his goal of travelling to Afghanistan to kill Americans, Mr Davoub said.
Mr Davoub added Mr Ahmed, a naturalised US citizen originally from Pakistan, had lived in the US since 1993.
A resident of Ashburn in Virginia, Mr Ahmed allegedly conducted surveillance of Metro stations in Arlington, on the outskirts of Washington DC, on four occasions.
He was conspiring with people he believed were from al-Qaeda, the Department of Justice said on Wednesday.
BBC News
