The FBI and the U.S. Capitol Police arrested a Moroccan man Friday in downtown Washington after a lengthy investigation into an alleged plot to carry out a shooting spree and a suicide bombing at the Capitol.

Amine el-Khalifi, 29, was picked up while carrying an inoperable MAC-10 automatic weapon and a fake suicide vest provided to him by undercover FBI agents posing as al-Qaeda associates, U.S. officials said. They said he entered the United States when he was 16 and was living as an illegal immigrant in Northern Virginia, having overstayed his visitor’s visa for years. Khalifi was arrested in a parking garage on Constitution Avenue NW a few blocks from the Capitol following a year-long investigation, officials said.