The FBI and Capitol Police arrested a man Friday that they say intended to detonate a suicide bomb at the U.S. Capitol.
All across the grounds of the U.S. Capitol, police kept watch, wary after officials said they had foiled a plot - a plot that would have struck at the heart of the U.S. government.
Police say 29-year-old Moroccan Amine El Khalifi - brought to a federal courthouse under heavy security - planned to shoot up the Capitol and then blow himself up.
The news took many of the tourists walking around the Capitol by surprise.
"Everything looked great, I mean, a perfect day for tourists and everything, so pretty scary," said New Zealand Tourist Hank Wang.
The arrest is the result of what authorities describe as an extensive investigation, during which El Khalifi lived illegally in nearby Virginia.
Officials say El Khalifi wanted to join a terrorist group and thought he was working with al-Qaida. What he did not know, officials say, was that his suicide vest, filled with explosives, had come from undercover officers and could not go off.
The arrest came on a day when lawmakers were in session and tourists flocked to the Capitol. But officials say no one was ever in any danger.
Still, some of those touring the Capitol had a feeling something was wrong.
"A lot of black vehicles were coming and going, quickly. And just that the police that were on their feet, were very alert and there were a multitude," said Florida tourist Kendall Clark.
Lynn and Steve Watson came in from the central state of Wisconsin.
"I think it's frightening. It's scary," said Lynn Watson.
"Quite honestly, I'm very surprised. I don't think we should have any of this happening, period," said Steve Watson.
Investigators say El Khalifi visited the Capitol numerous times in recent weeks to prepare for his would-be assault. If convicted, he faces life in prison.