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

Najibullah Zazi, 24, a Colorado airport shuttle driver and a legal U.S. resident born in Afghanistan, has been indicted by a grand jury on a charge of conspiracy use weapons of mass destruction.

I'd like to stop this rush to judgment because what I have seen so far does not amount to a conspiracy, defense attorney J. Michael Dowling told reporters outside the courthouse.

Prosecutors allege Zazi took a bomb-making course at an al Qaeda training camp in Pakistan, had bomb-making notes on his laptop, and acquired precursor materials for bombs similar to those used in the 2005 London attacks by loading up on acetone and hydrogen peroxide at beauty supply stores.

It is not against the law to buy the materials, Dowling said.

Law enforcement experts have called the suspected conspiracy, if proven, one of the most significant security threats in the United States since the attacks of September 11, 2001.

The Federal Bureau of Investigation is investigating other suspects it believes may have aided Zazi in acquiring the chemicals and consulted with him on how to make explosives.

Zazi faces a maximum penalty of life in prison if convicted. He has repeatedly denied wrongdoing in media interviews and through his lawyers.

The issue of al Qaeda training in Afghanistan and Pakistan is central to the ongoing debate about whether U.S. President Barack Obama should send more U.S. troops to Afghanistan, which the U.S. invaded in 2001 after the September 11 attacks.

Vice President Joe Biden has proposed a shift in the U.S. mission to concentrate on attacking al Qaeda targets that are primarily in Pakistan, using Predator drone missiles and other tactics.