The investigation started months ago, when the FBI noticed an email message: A man in the Chicago suburbs was using an account to distribute chatter about violent jihad and the killing of Americans.
Two undercover agents reached out and began to talk to him online. In May, they introduced him to another agent who claimed to be a terrorist living in New York.
The operation ended Friday night, an affidavit describing it says, when the man was arrested and accused of trying to detonate what he believed was a car bomb outside of a Chicago bar. Prosecutors said an undercover agent gave Adel Daoud, a U.S. citizen from the suburb of Hillside, a phony car bomb and watched him press the trigger.
The U.S. Attorney's Office in Chicago, which announced the arrest Saturday, said the device was harmless and the public was never at risk. Daoud, 18, is due to make an appearance in federal court Monday morning on charges of attempting to use a weapon of mass destruction and attempting to damage and destroy a building with an explosive.
"We don't even know anything. We don't know that much. We know as little as you do," a woman who answered the phone at his home and identified herself as his sister, Hilba, said Saturday. "They're just accusations. ... We'd like to be left alone."
The FBI often uses similar tactics in counterterrorism investigations, deploying undercover agents to engage suspects in talk of terror plots and then provide fake explosive devices.
In 2010, a Lebanese immigrant took what he thought was a bomb and dropped it into a trash bin near Chicago's Wrigley Field. In a 2009 case, agents provided a Jordanian man with a fake truck bomb that he used to try to blow up a 60-story office tower in Dallas.
This operation unfolded much like the others. After Daoud began talking to the undercover agents, an affidavit says, the third agent and Daoud met six times in the suburb of Villa Park over the summer and exchanged messages. Daoud then set about identifying 29 potential targets, including military recruiting centers, bars, malls and tourist attractions in Chicago, the document said.
After he settled on a downtown bar, he conducted surveillance on it by using Google Street View and visiting the area in person to take photographs, the affidavit said. The document does not identify the bar, but says he told the agent it was also a concert venue by a liquor store.
"It's a bar, it's a liquor store, it's a concert. All in one bundle," the document quotes him as saying. It said he noted the bar would be filled with the "evilest people ... kuffars." Kuffar is the Arabic term for non-believer.
Shortly after 7 p.m. Friday, the affidavit said, Daoud met with the undercover agent in Villa Park and they drove to downtown Chicago, where the restaurants and bars were packed. They entered a parking lot where a Jeep Cherokee containing the phony bomb was parked, the document says.
Daoud drove the vehicle and parked it in front of the bar, then walked a block away and attempted to detonate the device by pressing a triggering mechanism, the affidavit says. He was then arrested.
A neighbor, Harry Pappas, said that a dozen unmarked cars drove up to the family's house on Friday night and several agents went inside. On Saturday, no one answered the door of the family's two-story home, which had a well-kept garden in the yard and a basketball hoop in the driveway. The house faces a Lutheran church; a Greek Orthodox church also is nearby.
Pappas said he was shocked by the arrest, calling Daoud's parents "wonderful" people and him a quiet boy who played basketball in the driveway with friends.
"I heard maybe he had a little trouble in school," Pappas said. "He was quiet, didn't talk much, but he seemed like a good kid."
Pappas said Daoud spent a lot of time at home and that months would go by sometimes before the teen would surface.
"But I was never suspicious," he said.
Prosecutors said Daoud was offered several chances to change his mind and walk away from the plot.
The affidavit said Daoud was active in jihadist Internet forums and was accessing articles written by Anwar al-Awlaki, the U.S.-born radical cleric who became a key figure in the Yemen-based al-Qaida offshoot known as al-Qaida in the Arabian Peninsula.
Al-Awlaki was killed in a U.S. drone strike in Yemen last year.
The FBI says he also was searching online for information on making bombs and reading "Inspire," the English-language online magazine published by Al-Qaida in the Arabian Peninsula.
In his conversations with the undercover agent, Daoud explained his reasons for wanting to launch an attack, saying the United States was at war "with Islam and Muslims," the affidavit said.
According to the document, he said he was trying to recruit others and that he was confronted by leaders of his mosque who warned he should stop talking about jihad. The affidavit said Daoud's father also had been informed that Daoud was debating jihad and told Daoud to stop talking about it.
Daoud also told the agent he wanted an attack that would kill many people, the document said.
"I want something that's gonna make it in the news," he said, according to the affidavit. "I want to get to like, for me I want to get the most evil place, but I want to get a more populated place."