The accomplice of Oklahoma bomber Timothy McVeigh has been found guilty of 161 murder charges.
Nichols could now face death by lethal injection
Terry Nichols was already serving a life sentence for his role in the 1995 bombing, but prosecutors were seeking the death penalty.
The verdict came only five hours after the six-man, six-women jury
The same panel will now decide whether Nichols will face the death penalty or remain in prison.
Legal experts say Nichols faced trial again because many felt the original life sentence against him, decided by a federal court, was too lenient.
On Monday prosecutors told the state court in McAlester, Oklahoma, that Nichols had a greater part in planning the attack than McVeigh, who was executed in 2001 for setting off the blast.
In closing arguments on Monday, prosecutors said there was an "avalanche of evidence" against Nichols, adding that he had financed the blast and obtained key components of the bomb.
The attack was one of the deadliest on US soil in peacetime
"He contributed more than Timothy McVeigh," said prosecutor Lou Keel.
"It was him, he had the money."
Defence lawyers argued that McVeigh set up Nichols to take the blame for the roles of other unknown co-conspirators.
As well as 160 capital murder charges, Nichols was charged with conspiracy, first-degree arson and the murder of an unborn child.
Nichols was already serving a life sentence, having been convicted in 1997 of conspiracy to bomb the Alfred P Murrah building and the manslaughter of eight federal officials among the 168 people killed.