AURORA, Colo. -- The CALL7 Investigators have learned that FBI agents have visited Denver-area landscaping suppliers and companies where terrorism suspect Najibullah Zazi either purchased material or inquired about employment.

Zazi, who lived in Aurora, has been charged with conspiracy in a plot to attack the United States and is now in a federal holding facility in New York.

The FBI told 7NEWS it now has about 500 agents and law enforcement officers working on this case, just in Colorado.

It's now clear that some agents are visiting landscape suppliers who may have access to large quantities of ammonium nitrate fertilizers. That fertilizer was part of the truck bomb used by Timothy McVeigh and Terry Nichols in Oklahoma City.

An informed source associated with a Denver-area landscape supplier told CALL7 Investigator Tony Kovaleski that Zazi called during the first week of August asking about a job.

The source said the company does not want its name revealed but confirmed it had been contacted by the FBI.

Zazi had left a message with the company, but representatives did not return the call as there were no existing openings.

This latest information makes it clear the FBI is combing through Zazi's phone records and the source told Kovaleski the records include contacts to a variety of landscape material providers.

Kovaleski also spoke with the owner of a second company, Big Horn Landscape Materials, who confirmed that he, too, was contacted by the FBI.

The owner, Mike Maroney, told Kovaleski that federal investigators wanted to know if Zazi had bought fertilizer after discovering a credit card transaction for $205.90 in mid-August.

The owner informed the FBI and Kovaleski that Zazi bought 100 pounds of flagstone.

The FBI told 7NEWS it is not commenting on any part of this ongoing investigation and would not confirm the details uncovered by the CALL7 Investigators.

Our CALL7 Investigators are continuing to break new leads on this case. If you have any information about this case, or have a tip you want us to follow up on, contact CALL7 Investigator Tony Kovaleski at 303-832-0172 or e-mail him at 7newsinvestigates@thedenverchannel.com.