WASHINGTON — A fourth suspect in a disrupted plot to bomb New York City subways has been arrested in Pakistan, law enforcement officials said Monday.

The principal suspect in the case, Najibullah Zazi, has pleaded guilty and is cooperating with investigators, key developments that prosecutors hope will help them trace the plot back to its roots in Pakistan, where Zazi and former friends from high school allegedly traveled in 2008 to seek terror training.

After returning to the United States, the plotters allegedly hoped to detonate bombs on trains at two of the city’s biggest subway stations: Times Square and Grand Central Terminal, according to the two officials.