PRINCETON – A suspicious package found outside an eastern Minnesota high school Wednesday contained an incendiary device, the school district’s superintendent said.

Princeton schools Superintendent Rick Lahn said he learned that in a meeting with Police Chief Brian Payne on Wednesday afternoon.

“He said it was some kind of incendiary device, but it’s being investigated now and they’re taking a look at it,” Lahn said.

The package, which Lahn said was discovered by a custodian outside the school, was one of three found in Princeton on Wednesday morning.

The first was found before 6 a.m. behind a post office by an employee there, Payne said. Police later received calls from the high school and a public utilities building where the third package was found, he said.

The contents of the other two packages aren’t known. Payne and other law enforcement officials declined to say what was in any of them, citing the ongoing investigation. Payne wouldn’t give other details on the packages either, except to say they were similar.

Lahn closed the district’s schools and sent its 3,500 students home as a safety precaution but said classes would resume Thursday and this weekend’s homecoming activities would go on as scheduled.

He said school officials decided to evacuate the schools on the south side of town out of concern for student safety. After the fire department swept the schools on the north side to make sure they were safe, students were bused there, and officials arranged for more buses to take them home. He said the last student was delivered home before 12:30 p.m. Teachers and staff were also sent home, and activities for the remainder of the day were canceled.

Bob Schmidt, a special agent with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, said investigators were following up on leads and conducting interviews.

“There is no danger to the public,” Schmidt said.

Officials gave the all-clear late Wednesday morning after law enforcement officers and explosive-sniffing dogs combed the town of about 4,500 people about 50 miles north of Minneapolis.

“The entire town was searched for suspicious devices with negative results,” Schmidt said.

Along with the Princeton Police Department and ATF, agencies joining the investigation included the FBI, the St. Paul and Crow Wing County bomb squads, the Sherburne and Mille Lacs County sheriff’s departments, and postal inspectors, Schmidt said.