A superseding indictment unsealed Tuesday charged fugitive Canadian citizen Ferid Imam with a variety of offenses in a plot to blow up New York subways. Ringleader Najibullah Zazi was arrested in September 2009 after driving from Colorado to New York in hopes of carrying out a "martyrdom operation" for al-Qaida.

Attorney General Eric Holder called the plot "one of the most serious terrorist threats to our nation since September 11th, 2001 … it could have been devastating."

The charges in the Eastern District of New York court include providing material support to al-Qaida; aiding and abetting the terrorist training of Zazi and co-conspirators, Zarein Ahmedzay and Adis Medunjanin.

According to the indictment, Imam helped the three plotters receive military training from al-Qaida when they traveled to Pakistan in 2008. Zazi, Ahmedzay and Medunjanin later returned to the United States to carry out their plot of bombing American targets on behalf of the terrorist organization.

This plot was uncovered and disrupted by law enforcement authorities in September 2009. Zazi pleaded guilty to his role in the plot on Feb. 22, 2010, while Ahmedzay pleaded guilty on April 23, 2010. Canadian authorities issued similar charges against Ferid, a former Manitoba resident, Tuesday. He remains at large in Pakistan.