A disgruntled ex-con accused of plotting to blow up the federal courthouse in downtown Chicago (search) was arrested with a pickup truck containing 1,500 pounds of fertilizer he allegedly thought could be used to make a powerful bomb, authorities said.

Gale William Nettles (search), 66, planned to sell the chemical to terrorists so they could blow up the Dirksen federal building (search), U.S. Attorney Patrick J. Fitzgerald said in announcing the charges Thursday.

But everyone else involved, including the "terrorists" and the people who sold him the fertilizer, were cooperating witnesses or federal agents, Fitzgerald said. Authorities said Nettles thought the chemical was ammonium nitrate, the farm fertilizer used to blow up the Oklahoma City federal building.

Nettles had been under nearly constant surveillance since he walked out of a Mississippi prison late last year, authorities said. A felon who had been incarcerated with Nettles at the Mississippi prison tipped authorities about the plot.

The gray-haired Nettles appeared in court Thursday afternoon and Federal Magistrate Morton Denlow ordered that he remain in custody until a Tuesday detention hearing. Defense attorney John Theis declined to comment.

The investigation began in August 2003 while Nettles was serving time for counterfeiting at a federal prison in Yazoo City, Miss. It was there that Nettles asked a government informant if he could help him obtain ammonium nitrate, according to the criminal complaint.

Federal agents set up a sting after Nettles was released in October of 2003. Undercover agents on one end posed as terrorists intending to buy the bomb and carry out the plot, while undercover agents on the other end delivered the non-dangerous fertilizer to Nettles.

Nettles had asked cooperating witnesses about contacting Al-Qaeda or Hamas, Fitzgerald said.

According to the complaint, Nettles met July 25 with an undercover agent he thought was a member of a terrorist group. In a recorded meeting, Nettles said he had a half ton of ammonium nitrate in New Orleans that he could have in Chicago in two days and that he had a target in mind — the U.S. courthouse downtown, the complaint said.

Nettles told one undercover agent he could make a 3,000-pound fertilizer bomb and explained how to detonate it at a courthouse loading dock, according to the criminal complaint. In comparison, Timothy McVeigh used 4,800 pounds of ammonium nitrate to blow up the Oklahoma City federal building in 1995, killing 168 people.

According to the complaint, Nettles wanted to bring down the Dirksen building around 10 a.m. or 11 a.m., when judges would be present, and destroy two city blocks of downtown.

On Wednesday, an undercover agent delivered 500 pounds of the inert fertilizer to Nettles at a storage warehouse, the complaint said. The following morning, it said, the agent drove the pickup with an additional 1,500 pounds of fertilizer to a Chicago park and met Nettles.

Nettles was arrested at that park shortly afterward when he met the undercover agents posing as terrorists and accepted $10,000 from them, according to the complaint.