SUDBURY, Mass. -- Two shell casings found on school grounds sent Lincoln-Sudbury Regional High School into lockdown.

“I was in class and Mr. Carpenter came over the intercom and he told us that we had to stay in class, we couldn’t leave,” said Kelly Cronin, freshman.

For 2 and a half hours police placed Lincoln-Sudbury Regional High School in “safe mode” after a teacher found two .22-caliber spent casings on the floor of a classroom.

“They had us locked in our rooms and we couldn’t leave because they had the cops patrolling and checking out the area. And then they searched our bags and then later on they searched the lockers,” said Bella, junior.

Police say they searched 1,600 backpacks and then dismissed the students and told them not to stop at their lockers.

After students were gone police searched 3,000 lockers, but they didn't find a weapon or additional ammunition.

The heightened alert was expected, especially after John Odgren, who was a student there, was convicted of stabbing to death a fellow student in a bathroom at the high school in 2007.

As for Monday’s event, students say it was more inconvenient than frightening, and parents think the school did everything possible to keep students safe.

“They just found the shells and they said that they were taking certain precautions,” said Sue Cronin, parent. “I feel fine about it. It was okay. They took the steps that were necessary.”

The superintendent and principal at the school issued this email to parents: "Weapons of any sort should never be in school... Be it a small knife on a keychain or today's shell casings, we take these things seriously. We have to."

A police investigation is ongoing.

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