Staff Sgt. Paul Cimino, right, a master gunner with the 1-150th Assault Helicopter Battalion, New Jersey Army National Guard, gives the thumbs-up after securing M240B machine gun ammunition during an aerial gunnery training mission at the 177th Fighter Wing's Detachment 1, Warren Grove Gunnery Range, N.J., April 15, 2016.

AUSTIN, Texas — A malfunctioning retaining pin led to a box of ammunition dropping from an Army aircraft onto an El Paso elementary school in May, an investigation from the 1st Armored Division found.

The UH-60 Black Hawk helicopter, part of the division’s combat aviation brigade at Fort Bliss in El Paso, was flying a training mission with a modified “ammo can” on board, according to officials at the base. The clip that locks the ammo can into place had not been modified and it malfunctioned in air, dropping the can from the aircraft. The incident occurred May 17 at about 3:45 p.m. after the school day had ended and no one was injured.

No one was reprimanded for the incident because it was a mechanical issue, said Lt. Col. Crystal X. Boring, public affairs director for the division and Fort Bliss.

“Based on recommendations from the investigating officer, the aviation brigade commander immediately changed the flight path of future aviation missions,” she wrote in an emailed statement. “We have full faith and confidence in the aviation brigade leadership and their ability to constantly seek methods to improve flight operations and ensure an incident like this doesn’t happen in the future.”

The investigation also said the retaining pins in Black Hawks will need to be modified, and the brigade will only fly with the modified ammo cans over ranges in the meantime. The brigade has 57 Black Hawks. Whether this is an issue that affects all the Army’s Black Hawks is still under investigation.

Parkland Elementary School, located in northeast El Paso near Biggs Army Airfield, is part of the Ysleta Independent School District. In a news release in May, district officials said the can fell into a classroom, creating a hole in the roof and a power outage to a portion of the building.

Since then, all necessary repairs have been made. The final cost of repairs was unavailable.

thayer.rose@stripes.com

Twitter: @Rose_Lori