The Afghan-born man at the center of a U.S. anti-terrorism probe pleaded not guilty on Tuesday to allegations he plotted a bomb attack in the United States, and a federal judge ordered him held without bail.

Prosecutors accuse Najibullah Zazi, 24, a Colorado airport shuttle driver and a legal U.S. resident born in Afghanistan, of plotting bomb attacks in the United States. He is accused of conspiring to use weapons of mass destruction - homemade bombs. He and others allegedly bought chemicals at beauty supply companies to cook up a poor man's explosive known as triacetone triperoxide, or TATP.