Zazi, equipped with his supplies, then drove to New York, where he arrived Sept. 10 to undertake the suicide bombing as soon as materials were ready, he said in court.
Instead, Zazi began suspecting that police were watching him, so he got rid of the materials and flew back to Colorado, where he was arrested Sept. 19.
The guilty plea was the latest twist in a case that FBI Deputy Director John S. Pistole said Monday "illustrated the evolving nature of the terrorist threat," involving international travel and training on foreign soil to carry out attacks on domestic targets.
In January, two more men who were classmates of Zazi when he attended high school in Queens were arrested in New York in connection with the case.
One of them, Adis Medunjanin, has pleaded not guilty to conspiring to commit murder on foreign soil, a charge prosecutors say involves plans to fight the U.S. military in Afghanistan. The other, Zarein Ahmedzay, was charged with lying to authorities and has pleaded not guilty.
Zazi's father, Mohammed Wali Zazi, and a Queens imam, Ahmad Wais Afzali, also were arrested in connection with the case, but neither has been accused of direct involvement in the plot. Both have been freed on bail after pleading not guilty to charges that include lying to authorities.