Two men convinced of a larger conspiracy surrounding the Oklahoma City bombing have forced a grand jury probe into the topic.

Jury selection began Monday, thanks to 13,500 signatures collected by Charles Key and Glenn Wilburn on a petition to empanel a grand jury.Key, a state representative, and Glenn Wilburn, an accountant who lost two grandsons in the blast, said they have a profound distrust for the official explanation of the blast.

Timothy McVeigh awaits execution for bombing the Alfred P. Murrah federal building on April 19, 1995, and Terry Nichols faces a federal trial beginning Sept. 29.

The federal indictment alleged they plotted the bombing with "others unknown." But federal authorities now doubt a larger conspiracy. And they vehemently deny any prior knowledge of the attack, as Key and Wilburn also allege.

Robert Macy, the Oklahoma County district attorney who will advise the grand jury, already promised to file state murder charges against McVeigh and Nichols. Macy does not need a grand jury to bring charges.

He initially opposed the petition, but now says he hopes the panel will "find out what the truth was in the Oklahoma City bombing, if there is any additional evidence."

Oklahoma Attorney General Drew Edmondson, however, called the grand jury investigation a waste of time and taxpayer money.

"The notion that it can learn something that the FBI was unable to learn, is, I think, ludicrous," Edmondson said. "The witnesses that Mr. Key is talking about, we know who they are, we know what they have to say. That doesn't get us any closer to knowing the truth of it, hearing them say it again."

The petition names seven witnesses who have said they saw at least one other person with McVeigh in Oklahoma City on the day of the bombing, which killed 168 people. None of the witnesses was called before the federal grand jury primarily concerned with indicting McVeigh and Nichols.

It's not known how long the grand jury might stay in session, and it is possible Wilburn may not live to see the conclusion. He has inoperable pancreatic cancer.

If it returns no additional indictments, the panel's oath of secrecy would keep any new evidence from the public eye.