Terror suspect Najibullah Zazi reportedly had enough explosives to kill scores of people in the city.
Former FBI experts told the Associated Press Friday that the potential threat was on scale with the transit bombings in London and Madrid.
Zazi is accused of buying large quantities of hair dye and nail polish remover to make explosives.
Experts told the AP that Zazi likely wanted to make bombs out of flour and hydrogen peroxide and put them into backpacks, borrowing a tactic used overseas. It is not clear whether Zazi was able to make or test any bombs.
He is being held without bail and faces charges to conspiring to use weapons of mass destruction. He has denied any wrongdoing and pleaded not guilty to the charges.