Zazi is to appear Thursday in court in Denver on a count of lying to terrorism investigators. The new charge of conspiracy to use weapons of mass destruction was filed in New York.
The two-page indictment offers few details, but a separate document - a government motion seeking to deny bail to the 24-year-old Afghan immigrant - lays out evidence gathered by investigators.
The document says that on Sept. 6 and 7, Zazi tried on multiple times to communicate with another individual "seeking to correct mixtures of ingredients to make explosives."
"Each communication," the papers say, was "more urgent than the last."
On those days, Zazi rented a suite at a hotel in his hometown of Aurora, Colo., authorities charge. The room had a kitchen, and subsequent FBI testing for explosives and residue in the suite found the presence of residue in the vent above the stove.
Authorities plan to transfer him to the federal court in the New York borough of Brooklyn to face the new charge.
Zazi has publicly denied any terrorist plotting. Counterterrorism agents fear he and others may have been planning to detonate homemade bombs on New York City commuter trains.
Zazi's father and a Queens imam have also been charged with lying to investigators in the case.