Najibullah Zazi, seen here in Denver on Sept 17, 2009, was arrested for his alleged role in a plot to launch an attack in New York City using homemade explosives. (Ed Andrieski/Associated Press)

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 co-operating 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 travelled 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.

The officials spoke on condition of anonymity because they were not authorized to discuss details of the investigation. The new details in the case were first reported in Monday's editions of the New York Daily News.

Federal prosecutors have already said publicly they were seeking other suspects overseas. Three suspected members of the plot have been charged in the U.S. The two officials say a fourth person was arrested in Pakistan weeks ago in connection with the case.

Officials would say little else about the fourth person, other than the suspect is a citizen of Pakistan.

Both officials cautioned the person may not ultimately be charged with playing a role in the bomb plot.

Not guilty pleas

Zazi, a Colorado airport van driver, admitted that he tested bomb-making materials in a Denver suburb before travelling by car to New York intending to attack the subway system to avenge U.S. military involvement in Afghanistan.

The two other men suspected of direct roles in the plot, Adis Medunjanin and Zarein Ahmedzay, have pleaded not guilty to charges they sought to join Zazi in what prosecutors described as "three co-ordinated suicide bombing attacks" on Manhattan subway lines.

The alleged attacks were timed for days after the eighth anniversary of the Sept. 11 attacks.

Prosecutors say the attacks were modelled after the July 2005 bombings on the London transit system. Four suicide bombers killed 52 people and themselves in an attack on three subway trains and a bus in London.

The alleged New York plot was disrupted in early September when police officials stopped Zazi's car as it entered New York.

Last month, an Afghanistan-born imam linked to the suspects pleaded guilty to lying to the FBI when asked about the men.