After one of its eight engines inexplicably fell off during mid-flight earlier this week, the B-52 Stratofortress was able to land safely. The Air Force said that the engine loss was due to “catastrophic engine failure.” Uh, oh.

Thankfully, the five people aboard the aircraft at the time of the incident were not injured. Defense News reports that the Air Force is still investigating the cause of the problem, but

it appears that the engine began breaking down from the inside, eventually cracking the protective casing around it and detaching from the plane, Air Force Secretary Deborah Lee James said in an exclusive interview.

The head of Air Force Global Strike Command, Gen. Robin Rand, told Defense News in a separate interview, “The engine didn’t just fall off. The engine had a failure inside the engine, and it shelled itself.”

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Officials don’t think that the engine, located under the aircraft’s wings, was improperly mounted. Instead, what is believed to have happened is that one of the engines had an internal problem and caused the protective structure on the outside to crumble.

The engine’s wreckage was found in a riverbed about “25 nautical miles” away from the Minot Air Force Base in North Dakota, but it remains unclear how or when it will be recovered.

It’s a good thing it has so many backups.