WASHINGTON >> A 29-year-old Moroccan man was arrested Friday near the U.S. Capitol as he was planning to detonate what he thought was a suicide vest, given to him by FBI undercover operatives, said police and government officials.

Amine El Khalifi of Alexandria, Va., was taken into custody with an inoperable gun and inert explosives, according to a counterterrorism official.

El Khalifi expressed interest in killing at least 30 people and considered targeting a building in Alexandria and a restaurant, synagogue and a place where military personnel gather in Washington before he settled on the Capitol after canvassing that area a couple of times, the counterterrorism official said. During the investigation, the official said, El Khalifi went with undercover operatives to a quarry in the Washington area to detonate explosives.

El Khalifi came to the U.S. when he was 16 years old and is unemployed and not believed to be associated with al-Qaida. He had been under investigation for about a year and had overstayed his visitor visa for years, according to the counterterrorism official and a government official briefed on the matter who spoke on a condition of anonymity because the investigation is ongoing.

Police are close to arresting one of his associates on charges unrelated to the terror conspiracy, the counterterrorism official said. The associate was said to also be a Moroccan, living here illegally. Police are investigating others El Khalifi associated with, but not because they believe the associates were part of a terror conspiracy, the official said.

Two people briefed on the matter told The Associated Press he was not arrested on the Capitol grounds, and the FBI has had him under surveillance around the clock for several weeks. They spoke on a condition of anonymity because they were not authorized to speak publicly.

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Associated Press writers Eileen Sullivan, Matt Apuzzo, Adam Goldman, Laurie Kellman and Eric Tucker contributed to this report.