The indictment, returned in Ohio, charges all eight in two separate conspiracies, one to provide material support to terrorists and a second to commit murder on federal government territory and to murder a federal government official.
It remains unclear from the court records how close the would-be attackers could have come to being able to carry out the plan had it not been foiled.
According to the new indictment, the plot began in May, when the group began amassing money, firearms, ammunition, body armour, explosives, drones, medical equipment, communications equipment and other items.
Law enforcement officials learned about a possible threat to President Donald Trump's UFC cage-fighting show, four days before it was scheduled to take place.
The Justice Department announced federal charges against seven people last month from across the country, including from Ohio, Missouri, Washington, Nebraska and California. Officials said the group members harboured fringe conspiracy theories and hoped the attack would destabilise the government.
One of the defendants told investigators that they planned to fly explosive-laden drones into the event and then shoot panicked crowd members as they fled, according to a federal affidavit.
Tycen C Proper 19, of Danville, Ohio, and four others were arrested and charged in Missouri, Nebraska and California the weekend of the cage-fighting event, called Freedom 250. Two more defendants were charged and arrested by the FBI about a week later in Washington and Missouri. The Justice Department said the eighth man was charged this week.
He is Chandler D Scaggs, 21, of Chapmanville, West Virginia, who was taken into custody in that state. Scaggs was allegedly assigned to be one of the snipers in the plotted attack, according to an affidavit.
The affidavit said Scaggs was apparently to be picked up by Proper and taken to Washington but lost contact with him after Proper was arrested, as were the others. Scaggs allegedly signalled to the group that he was still willing to participate in the attack and arranged to travel to the event with another co-conspirator.
Conspiring to provide material support to terrorists is punishable by up to 15 years in prison, and conspiring to commit murder carries a penalty of up to life in prison.
Federal prosecutors allege the group planned to murder Trump, Vice President JD Vance, other federal officials, Israeli Prime Minister Benjamin Netanyahu, trillionaire businessman Elon Musk and "other high value targets" at the event.
