"He said it was some sort of incendiary device, but it is now investigating and is taking a look," Lahn told The Associated Press. "And I could not give further details. He just said he was very suspicious and it contained some explosive material. I do not think it was a large device, but I really do not know what kind of damage that could have done."

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

The first was found before 6 am behind a post office by an employee there, said Payne. Later, police received calls from high school and a public service building where he found the third package, he said.

The contents of the other two packages are not known. Payne and officials of law enforcement refused to say what was in any of them, citing the ongoing investigation. Payne did not give further details on the packages or, except to say that were similar. The police chief also declined to comment on whether investigators had made no arrests and had no suspects.

Lahn district schools closed and sent its 3,500 students from home as a security precaution but said it will resume classes on Thursday and activities back home this weekend would be scheduled.

He said that school officials decided to evacuate the schools on the south side of town because of concerns about the safety of students. After the fire department sweep of schools in the north to ensure they were safe, the students were bused there, and officials prepared for more buses to take them home. He said the last student was given a home by 12:30 pm Teachers and staff also were sent home, and activities for the rest of the day were canceled.

Bob Schmidt, a special agent with the Bureau of Alcohol, Snuff, Firearms and Explosives, said investigators were monitoring the tracks and interviews.

"There is no danger to the public," said Schmidt. "All these elements have been secured and is resuming business in the city of Princeton."

Officials gave the go-ahead on Wednesday evening after police and bomb-sniffing dogs combed the town of about 4,500 people 50 miles north of Minneapolis.

"The whole town was searched for suspect devices, with negative results," said Schmidt.

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

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