The IRS told MSNBC "an unknown substance" was found, and reports suggested that white powder was found in an envelope.
Early reports said that two people had been taken from the building in stretchers, but the FBI, which is leading the investigation, told the AP those people had medical emergencies that did not "appear to be related to this incident."
According to a recent Salt Lake Tribune story, the IRS employs at least 6,500 people at the Ogden office.
As of late Monday afternoon, several emergency vehicles were reported to have left the scene.
The incident occured 11 days after Joe Stack flew his plane into an IRS building in Austin, TX, leaving behind an anti-IRS screed.
Late Update: According to one report, a U.S. Postal Inspection Service official said the materials found were not hazardous.