HELENA, Mont. — An envelope containing a suspicious powder burst open today while being handled by state workers in a Montana government office.

Three Department of Labor workers and a Helena city police officer were isolated while a state lab tested the substance. Other workers on the third floor of the Walt Sullivan building were relocated elsewhere for the day.

The state said the envelope burst, as if there were a triggering device to distribute the powder. The contents of the envelope were otherwise described as a standard Department of Labor form.

A state spokeswoman said there was no indication who sent the letter to the state.