A nine-year-old girl took a hand grenade to an elementary school in Morelos, a state in central Mexico, authorities said.
Administrators at the school in the city of Cuautla called the emergency response number and told operators they had an explosive device.
Army troops, law enforcement agents from various federal, state and municipal departments, firefighters and emergency management office personnel went to the Otilio Montano school.
School administrators handed over the grenade to emergency personnel.
Several fourth-grade students told teachers that a classmate had a bomb, prompting school personnel to search students' belongings, school administrators said.
The grenade, which still had the fuse in it, was found in the girl's backpack, school administrators said.
Investigators are trying to find out where the grenade came from and how the girl ended up with the military weapon.
Army specialists are working to determine if the grenade is active.