A letter with a granular substance was sent to the Colorado History Museum on Wednesday morning, mimicking four other letters sent to state offices this week, Denver fire Lt. Phil Champagne said.
The substance found inside the envelope was probably sugar or an artificial sweetener, he said.
About 100 employees at the museum, 1300 Broadway, left the building on their own before firefighters arrived to examine the substance.
The letter also contained threats, as did the four letters sent to three downtown offices and the Colorado Department of Agriculture building in Lakewood.
The U.S. postal inspector is investigating who sent the letters.
Although the powder in the envelopes is considered harmless, authorities say sending threats through the mail is a serious crime.
The letters were sent from locations in the Denver metro area, but that may not be an indicator of who sent them, authorities said.
Anyone with information is asked to call the U.S. postal inspector at at 303-313-5320. Press 4 to reach an investigator.
Staff writer Felisa Cardona can be reached at 303-954-1219 or at fcardona@denverpost.com.
