More details emerged Monday over an alleged attempt to set off explosives in the New York City subway system last year and a fourth suspect was reportedly arrested in Pakistan.

The New York Daily News reports an unidentified man will be extradited from Pakistan and will face trial in a Brooklyn federal court.

The man in charge of the plot, 25-year-old Najibullah Zazi, has cooperated with law enforcement officials and has said that he and two others planned to strap bombs to themselves aboard trains at the Times Square and Grand Central subway stations in mid-September of last year.

Zazi initially denied his involvement, but back in February, under the terms of a plea agreement, Zazi pleaded guilty to conspiracy to use weapons of mass destruction and supporting al Qaeda, which he said he received training from in Pakistan in 2008.

He faces life in prison without parole at his June sentencing, but his cooperation with authorities could earn him leniency.

He reportedly told prosecutors the plot was planned for the height of rush hour on some of the subway’s most congested lines.

The newly arrested, unidentified suspect’s role is still unclear.

But there’s nothing vague about Attorney General Eric Holder’s assessment of the busted terror plot.

“This is one of the most serious threats to our nation since September 11, 2001.”

(Source: WCBSTV / NY Daily News)