Students at the Duxbury Middle School were temporarily held on lockdown after police discovered a live round of ammunition Friday afternoon.

Police were called to the school after two shells were discovered, said Lt. Lewis Chubb of the Duxbury Police. One was a spent casing, but one was a live round, Chubb said. The bullets were different calibers, he added.

Students were asked to remain in their classrooms and police conducted a thorough search of the building using dogs and metal detectors, Chubb said, to make sure there was no weapon in the school.

“The item of concern is the live round,” he said. “We had to be sure it didn’t come in with a gun.”

Superintendent Susan Skeiber called the police procdure a semi-lockdown, and said School Resource Office Friend Weiler was called in to help the police and schools work together.

“The police came in and did a nice thorough search for us,” she said. Police searched backpacks with metal detecting wands and searched individual lockers with dogs. Skeiber said that considering that, the district did not enact any additional security procedures when school started again on Monday.

“I think everyone was fairly calm and felt things were handled appropriately,” Skeiber said.

Students were held in their rooms beyond the normal school dismissal time. They were released when police were satisfied there was no weapon in the school, Chubb said.

Skeiber said the lockdown did not affect the release of students from the Alden Elementary School, which is adjacent to the middle school, but the delay of buses from the middle and high schools delayed Chandler students by about half an hour.

Skeiber said Tuesday the schools still do not know who brought the bullet into the school. Chubb said the Police Department is still investigating to find out where the ammo may have come from.