JUNEAU -- Four school districts around the state have each received a suspicious envelope filled with an unknown substance this week.

In Juneau on Wednesday, a letter prompted the evacuation of the district's central offices. Also on Wednesday, Maudrey J. Sommer School, a K-12 facility in Tanana was shut down while a hazardous materials team based in Fairbanks flew approximately 100 miles northwest to investigate, according to the Fairbanks Daily News-Miner. Classes resumed Thursday in Tanana. In both cases, hazardous teams confirmed that there was no danger.

When a suspicious envelope was found in the offices of Chugach School District in south Anchorage on Tuesday morning, a slew of officials responded: FBI agents, a hazardous materials team and Anchorage police officers showed up, but the substance was benign.

Dr. Bernd Jilly of the Alaska Public Health Laboratories, which serves all of Alaska, said he has tested the substances that caused a scare in the Chugach and Juneau districts to confirm they were not infectious or toxic; he said his results were the same as emergency responders, and he passed the materials back to investigators. An envelope from Thorne Bay is en route for testing. Teri Willard is a clerk for Southeast Island School District in Thorne Bay, about 40 miles northwest of Ketchikan.

She said Lauren Burch, the district's superintendent, had forwarded her an email earlier in the week from the U.S. Department of Education, warning of mail with a Texas postmark. Burch plans a trip to the Longhorn State to recruit teachers, Willard said, but because of the warning, she was wary when she saw an envelope from Texas on Wednesday.

"We thought it was maybe a resume for the job fair, but the envelope felt like it could have something else in there," Willard said. "We wrapped it in saran wrap and called the number in the email."

Jilly said his office usually processes between six and 12 incidents involving suspicious mail each year. "It's a federal crime so we take it all seriously, but this is not an unusual occurrence," Jilly said.

FBI spokesman Special Agent Eric Gonzales said his Anchorage-based office is working with the post office, other law enforcement officials and state public health officials to figure out what's going on.

There have been reports around the country -- from Washington, D.C., to Boston to Houston -- of similar envelopes sent over the past week. Gonzales said the Anchorage office is working with FBI agents around the country where similar envelopes have turned up. He said he could not provide any more details until the investigation is complete.