OKLAHOMA CITY (AP) — A man accused of attempting to blow up a bank in downtown Oklahoma City has been indicted.

An Oklahoma City federal grand jury formally charged Jerry Drake Varnell, 23, this week of one count of attempted use of an explosive device.

The FBI arrested Varnell in August on a charge of attempting to detonate what he believed to be explosives in a van outside the BancFirst building. Court records show the FBI discovered Varnell's alleged plans last year and closely monitored Varnell for months before the arrest.

Authorities said an undercover FBI agent helped Varnell construct the explosives with inert materials. The FBI alleges that Varnell was upset with the U.S. government and wanted to retaliate.

"He wanted to make the biggest impact wherever he was going to place his bomb and, in his mind, this would have been the biggest impact in Oklahoma," said Raul Bujanda, FBI assistant special agent in charge.

A competency hearing for Varnell has been ordered. A competency exam is expected to be completed by Nov. 4.

Varnell's family has said he's a paranoid schizophrenic who has been in numerous mental hospitals since he was 16 years old. They allege FBI agents knew Varnell was mentally ill and have accused the agency of manipulating his mental illness to build a criminal case.

Varnell could face a maximum sentence of 20 years in prison and a mandatory minimum sentence of five years if convicted.