The official and other conspirators are being protected by the federal government "in a cover-up to escape its responsibility for the loss of life in Oklahoma," Nichols claims in a Feb. 9 affidavit.
Documents that supposedly help back up his allegations have been sealed to protect information in them, such as Social Security numbers and dates of birth.
The U.S. Attorney's Office in Utah had no comment on the allegations. The FBI and Justice Department in Washington, D.C., also declined comment.
Nichols does not say what motive the government would have to be involved in the bombing.
The affidavit was filed in a lawsuit brought by Salt Lake City attorney Jesse Trentadue, who believes his brother's death in a federal prison was linked to the Oklahoma City bombing. The suit, which seeks documents from the FBI under the federal Freedom of Information Act, alleges that authorities mistook Kenneth Trentadue for a bombing conspirator and that guards killed him in an interrogation that got out of hand.
Trentadue's death a few months after the April 19, 1995, bombing was ruled a suicide after several investigations. The government has adamantly denied any wrongdoing in the death.
In his affidavit, Nichols says he wants to bring closure to the survivors and families of the attack on the Alfred P. Murrah Federal Building, which took 168 lives. He alleges he wrote then-Attorney General John Ashcroft in 2004, offering to help identify all parties who played a role in the bombing but never got a reply.
Nichols is serving a life sentence at the U.S. Penitentiary Administrative Maximum Facility in Florence, Colo. McVeigh, who carried out the bombing, was executed in 2001.
McVeigh and Nichols were the only defendants indicted in the bombing. However, Nichols alleges others were involved.
McVeigh told him he was recruited for undercover missions while serving in the military, according to Nichols. He says he learned sometime in 1995 that there had been a change in the bombing target and that McVeigh was upset by that.
"There, in what I believe was an accidental slip of the tongue, McVeigh revealed the identity of a high-ranking FBI official who was apparently directing McVeigh in the bomb plot," Nichols says in the affidavit.
Nichols also says that McVeigh threatened him and his family to force him to rob Roger Moore, an Arkansas gun dealer, of weapons and explosives. He later learned the robbery was staged so Moore, who was in on the phony heist, could deny any knowledge of the bombing plot if the stolen items were traced back to him, Nichols claims.
He adds that Moore allegedly told his attorney that he would not be prosecuted in connection with the bombing because he was a "protected witness."
Moore could not be reached for comment Tuesday.
In addition, Nichols says McVeigh must have had help building the bomb. The device he and McVeigh built the day before the bombing did not resemble the one that ultimately was used, Nichols says, and "displayed a level of expertise and sophistication" that neither man had.