Undercover Federal Bureau of Investigation agents arrested an 18-year-old American man who tried to detonate what he was led to believe was a car bomb outside a downtown Chicago bar, federal prosecutors said Saturday.

Adel Daoud, a U.S. citizen from the Chicago suburb of Hillside, was arrested Friday night in an undercover operation in which agents pretending to be terrorists provided him with a phoney car bomb.

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 is charged with attempting to use a weapon of mass destruction and attempting to damage and destroy a building with an explosive.

Someone who answered a call to Daoud's home in Hillside on Saturday who said her name was Hiba and that she was Daoud's sister declined to discuss Daoud, the family or the arrest.

'We'd like to be left alone'

"We don't even know anything. We don't know that much. We know as little as you do," she said. "They're just accusations."

"We'd like to be left alone," she said.

No one answered the door of the two-storey brick home later Saturday.

A next-door neighbour said he was shocked by the arrest, describing Daoud as a quiet boy who played basketball in the driveway with friends and calling his parents "wonderful" people.

"I heard maybe he had a little trouble in school," said the neighbour, 78-year-old Harry Pappas. "He was quiet, didn't talk much, but he seemed like a good kid."

Pappas said he saw a dozen unmarked cars drive up to the house Friday night and several agents go inside.

The FBI began monitoring Daoud after he posted material online about violent jihad and the killing of Americans, federal prosecutors said.

In May, two undercover FBI agents contacted Daoud in response to the postings and exchanged several electronic messages with him in which he expressed an interest in engaging in violent jihad in the United States or abroad, according to an affidavit by an FBI special agent.

29 potential targets

Prosecutors say that after being introduced to an undercover FBI agent who claimed to be a terrorist living in New York, Daoud set about identifying 29 potential targets, including military recruiting centres, bars, malls and tourist attractions in Chicago.

He is accused of settling on a downtown bar and conducting surveillance on it using Google Street View and visiting the area in person to take photographs.

Describing the target to the agent, Daoud said it was also a concert venue and next to a liquor store, according to the affidavit.

"It's a bar, it's a liquor store, it's a concert. All in one bundle," the document quotes him as saying. The affidavit said he noted that the bar would be filled with the "evilest people ... all the kuffars are there." Kuffar is the Arabic term for non-believer.

Shortly after 7 p.m. Friday, Daoud met in the suburb of Villa Park with the undercover agent who claimed to be from New York, and the two drove to downtown Chicago, where the restaurants and bars were packed with workers ringing in the weekend on a pleasantly warm evening. According to the affidavit, they entered a parking lot where a Jeep Cherokee containing the phoney bomb was parked.

Daoud drove the vehicle and parked in front of a bar that was among the pre-selected targets, then walked a block away and attempted to detonate the device by pressing a triggering mechanism in the presence of the agent, according to the affidavit. He was then arrested.

The court documents do not identify the bar.

Didn't walk away

Prosecutors said Daoud was offered several chances to change his mind and walk away from the plot.

The FBI has used similar tactics in other counterterrorism investigations, deploying undercover agents to engage suspects in talk of terror plots and then provide them with fake explosive devices.

In a 2010 case, 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-storey office tower in Dallas.

The affidavit said the 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-Qaeda offshoot known as al-Qaeda in the Arabian Peninsula.

Al-Awlaki was killed in a U.S. drone strike in Yemen last year.

In his communications with one of the FBI agents about possible targets, Daoud allegedly said he wanted to carry out an attack that would kill a large number of people.

"I wanted something that's ... massive; I want something that's gonna make it in the news," he wrote, 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."