(NewsNation) — The U.S. Army confirmed to NewsNation that the reason they grounded an entire fleet of Chinook helicopters earlier this week was because incorrect parts were installed in some of the aircraft.

As NewsNation previously reported, 400 Chinook cargo helicopters were grounded, the Army said Wednesday, after fuel leaks caused a “small number” of engine fires. No injuries or deaths were reported, the Army said.

Army officials said they grounded the helicopters out of an “abundance of caution.”

This all happened after a piece of equipment in the engines was replaced with a wrong part during maintenance, according to the Army.

The part “was assigned an incorrect part number and was subsequently installed on some Chinook helicopters,” the Army said in a statement. “This error has been corrected.”

The Wall Street Journal reported that the part, called an O-ring, is suspected of causing seven fuel leaks, with four of them resulting in engine fires.

Chinooks have been relied on by the Army for nearly 60 years. They can carry up to 36 troops and loads of cargo, and are also used in search-and-rescue missions and during parachuting.

As of Saturday, the choppers were still grounded. NewsNation doesn’t have a timeline from the Army on when the aircraft will be back in action.

A private-sector engineer familiar with Chinooks previously told NewsNation’s “Rush Hour” Wednesday he hopes the Army solves the problem quickly.

“With the functionality and the capability of them, I think everybody is all-hands-on-deck at this point to get these things in the air as fast as we can,” said Rob Roedts, senior director of aviation technology and manufacturing with Columbia Helicopters.