WASHINGTON  - A naturalized US citizen from Pakistan pleaded guilty on Monday to joining what he thought was an al Qaeda plot to bomb the Washington area subway system and received a 23-year prison sentence.

Farooque Ahmed, 35, was arrested in October in a US government sting operation for conducting surveillance at subway stops and suggesting where to place explosives.

Ahmed, who moved to the United States in 1993, pleaded guilty to one count of attempting to provide material support to al Qaeda and one count of collecting information to assist in the planning of a terrorist attack on a transit facility, the Justice Department said.

Ahmed, who previously pleaded not guilty, changed his plea during a federal court hearing in Alexandria, Virginia.

As part of the plea deal, prosecutors dropped one count against Ahmed and agreed to the 23-year prison term, officials said. He had previously faced up to 50 years in prison.

US officials said the public was never in any danger and Ahmed's activities were closely monitored during the six-month sting operation in which government agents posed as al Qaeda operatives.

After his arrest, the FBI said Ahmed had told undercover agents that he had trained himself to use firearms in hopes of fighting US soldiers in Afghanistan.

Since the September 11, 2001, attacks, the Justice Department and FBI have faced criticism for such sting operations and questions over whether the suspects have been unfairly targeted and lured into plots.

Justice Department officials have defended them as legal and an important way to prevent another attack on US soil.

Other recent sting operations include a Somali-born teenager arrested in November in Portland, Oregon, on charges of joining what he thought was a plot to set off a bomb at a Christmas tree lighting ceremony.