FORT KENT, Maine — Agents from the Federal Bureau of Investigation were working with local law enforcement Tuesday night to determine who sent an unidentified white powder to Fort Kent Elementary School.

The powder came in an envelope with a letter and had a Rowlette, Texas, postmark.

Tim Doak, superintendent of AOS 95, said the letter was opened in the principal’s office around noon. The secretary opened it and found the powder and a letter that Doak said “said some derogatory things about the FBI.”

Doak said school officials immediately placed the envelope into a sealed zip-close bag, as protocol dictates. They then called police, who arrived a short time later.

The school was not evacuated.

“We kept people out of the office and saw that no one in the office had gotten sick,” he said. “It would have been different if this letter had been opened in a classroom or the cafeteria or a hallway or something. But it was in the principal’s office, and the cops told us pretty quickly that it did not appear suspicious.”

Fort Kent Police Chief Kenneth Michaud said FBI agents from Waterville drove up to examine the contents of the envelope.

“Six people came to the police department,” Michaud said Tuesday evening. “The tests showed that it does not look harmful at this time, but they still don’t know what it is. They are taking it to a lab in Augusta.”

Schools across the nation, including in Texas, Massachusetts and Rhode Island, have received similar envelopes with powder in them since Monday, according to The Associated Press. The contents have turned out to be cornstarch in several cases.

Doak said that the school would re-evaluate its protocol to see if anything needs to be changed, but he said that police told school officials they acted correctly by sealing it up and calling authorities.