WASHINGTON — A naturalized U.S. citizen from Pakistan was arrested Wednesday after he allegedly spent six months casing crowded Metro subway stations around the Pentagon in what prosecutors said was a plan “”to kill as many military personnel as possible”” by placing explosive devices on the trains and detonating them during rush hour.

Farooque Ahmed, a 34-year-old resident of Ashburn, Va., in the Washington suburbs, allegedly wanted to stage a series of coordinated bombings on the trains on behalf of al-Qaida terrorists whom he did not realize were actually FBI undercover operatives.

According to a federal indictment, Ahmed repeatedly scouted commuter traffic at several subway stations leading in and out of the Pentagon in Virginia, and reported his findings back to the undercover operatives.

So detailed was his surveillance, the indictment said, that Ahmed suggested that bombings between 4 and 5 p.m. would “”cause the most casualties”” and that explosives should be snuck aboard the trains on “”rolling suitcases.””

Ahmed, in a long beard and casual clothes, appeared in federal court on charges of attempting to provide material support to a foreign terrorist organization and collecting information to assist in planning a terrorist attack on a transit facility.

A former network planning engineer and computer scientist in New York, he said he could not afford a lawyer. He did not enter a plea in the case, where he could receive up to 50 years in prison.

Neil H. MacBride, the U.S. attorney in Eastern Virginia, called it “”chilling”” that someone was “”casing rail stations with the goal of killing as many Metro riders as possible through simultaneous attacks.””

The Ahmed case, which President Barack Obama was briefed on before the arrest, is the latest in a string of alleged bomb plots by so-called homegrown terrorists. David Kris, assistant attorney general for national security, said it “”underscores the need for continued vigilance against terrorist threats”” in the United States.

The nine-page indictment gave this scenario:

Ahmed met in April at a hotel in Dulles, Va., with an undercover operative posing as a “”courier”” for a terrorist organization. A month later he met at a hotel in nearby Sterling, Va., and told another operative that he “”might be ready to travel overseas to conduct jihad”” in January after he completed a pilgrimage to Saudi Arabia in November. He also agreed to “”watch and photograph”” the Arlington Cemetery subway station “”in order to obtain information about their security and busiest periods.””

Ahmed twice scouted the station in July, and made a “”recording of video images.”” He sent an e-mail signaling his surveillance was done. He gave a thumb drive containing the video to one of the undercover operatives, and agreed to assess security at two other stations — the Arlington Courthouse and the Pentagon City mall.

Ahmed also said he wanted to donate $10,000 to his “”brothers overseas,”” and would send the money in $1,000 increments “”in order not to raise any red flags.””

In August he scouted the other two stations, and in September gave a thumb drive of images from those stops to an operative.

Then on Sept. 28 in a lengthy meeting at a hotel in nearby Herndon, Va., Ahmed told an operative that “”between 4 and 5 p.m. would be the best time to stage an attack to cause the most casualties.”” He also suggested another station at Crystal City be hit.

He sketched diagrams of the stations, and suggested explosives be hidden inside “”rolling suitcases”” rather than more conspicuous “”backpacks.”” The indictment added that Ahmed told the operative he “”wanted to kill as many military personnel as possible.””