State fire officials sent a white powder substance to a lab for further testing after it was sent and opened early Tuesday afternoon at Dedham Middle School.
An administrative assistant opened the envelope, and alerted authorities. She was isolated, as was the school's office and foyer, Dedham Fire Chief William Cullinane said.
"We isolated and contained the letter at the desk that she was at,"Cullinane said.
Cullinane said officials hadn't determined what the substance was, but said it didn't pose a danger. A state lab could take up to two days to get definitive results on the substance.
"It was inconclusive, but I can tell you that we do not believe that there was anything hazardous in that envelope," Cullinane said during a press briefing.
Students were not put in lockdown during the event, but were kept away from the school's cafeteria and office.
"I must emphasize that the children were never in any danger and were kept in classrooms in order to allow all personnel to attend to the issue," school principal Debra Gately wrote in an email to parents. "At no time were any students in any type of danger."
Voting at the school in Tuesday's election also was not impacted.
The incident is similar to others that happened across New England Tuesday, including in Milford. A school official told Milford Patch that the substance was corn starch.
The letter sent to Milford had a return address from Texas, but Cullinane and State Fire Marshal Stephen Coan declined to comment on the envelope sent to DMS.
"The incident in Milford is consistent to what's been found here," Coan said during a press briefing. "We are not at liberty to release any detail as to the description of the envelope."
Coan's office has alerted local school districts and public safety departments about the envelopes and with procedures on how to handle one of the envelopes.
"It effects the educational system in these communities and other communities, and it spreads our resources throughout the state," Coan said.