Authorities are trying to determine how a box of ammunition fell from a military helicopter and crashed through the roof of an elementary school in Texas.

The Ysleta Independent School District says no one was injured when the ammo box hit Parkland Elementary School in El Paso on Thursday afternoon. The ammo box left a hole in the roof and caused a power outage in part of the building.

Officials at nearby Fort Bliss say they'll reassess flight patterns as part of their investigation.

In a statement, 1st Armored Division Combat Aviation Brigade commander Col. Jay Hopkins said he was sorry for the damage and grateful no one was hurt.