WASHINGTON (CNN) — A Moroccan man accused of plotting a suicide bomb attack on the U.S. Capitol pleaded guilty Friday afternoon to a charge of attempting to use a weapon of mass destruction against government property.

In the plea agreement, Amine El Khalifi, 29, agreed to a prison sentence with a maximum of 25 to 30 years. If he had been convicted in a trial he could have been sentenced to life behind bars.

U.S. District Judge James Cacheris set a sentencing date of September 14.

Amine El Khalifi came to the courtroom wearing a jail jumpsuit with the word “prisoner” stenciled on the back. He was not in restraints.

He appeared relaxed and politely answered the judge when asked to confirm his guilty plea and other questions. The judge accepted the plea and found him guilty.

El Khalifi was arrested February 17 as the result of an FBI sting operation.

According to a criminal complaint, El Khalifi accepted what he thought was a suicide vest containing explosives and a MAC-10 automatic gun. He put on the vest and was quickly taken into custody as he attempted to leave a parking garage and walk toward the Capitol building.

“Unbeknownst to El Khalifi, both the weapon and the bomb had been rendered inoperable by law enforcement,” the criminal complaint says. El Khalifi allegedly told an undercover FBI agent and another man assisting law enforcement agents that he intended to use the MAC-10 to “shoot people before detonating the bomb.”

El Khalifi allegedly thought he was working with al Qaeda, but a man referred to in court documents as “Yusuf” was an undercover agent. U.S. officials said El Khalifi had been closely monitored and the public was never in danger. The U.S. officials also said he was not connected to terrorists and was working alone.

U.S. Attorney Neil MacBride said at the time of El Khalifi’s arrest that he “devised the plot, the targets and the methods on his own.”

According to the affidavit, El Khalifi changed his mind about what to target and at various points had considered hitting a restaurant, a military installation and a synagogue. He finally decided to try to blow himself up in the Capitol, according to the court document, and said “he would be happy killing 30 people.”

El Khalifi came to the United States in June 1999 under a B2 visa, which allows visitors for tourism and medical treatment. But U.S. officials said El Khalifi’s visa expired later the same year and he has been in the country illegally ever since.