An E-8C Joint Surveillance Target Attack System aircraft suffered water damage to its radar antenna and other electrical systems after Northrop Grumman contract maintainers forgot to open up drain holes, according to an investigation report released Friday.

The accident investigation board report, released by Air Force Materiel Command, found that the maintainers at Northrop's Lake Charles, Louisiana, facility covered up three drain holes in the JSTARS' radome, a canoe-like structure located underneath the aircraft's fuselage, and failed to make sure they were cleared after washing the aircraft.

As a result, 4.5 inches of standing wash water and rainwater collected in the radome between June 4 and July 27 last year and caused an electrical current. This resulted in $7.35 million in "visible corrosion" and electrical damage to many of the antenna's components, including 240 circuit cards. Most of those electrical components had to be replaced.

The discovery of the problems led to the grounding of four JSTARS aircraft at the depot, including the water-damaged plane, Defense News reported last September. All four planes were assigned to the 116th Air Control Wing at Robins Air Force Base in Georgia.

Northrop's maintainers were required to make sure the drain holes weren't obstructed as part of the pre-flight inspection process, the report said. But pre-flight inspections were conducted four times last July, and signed off on by Northrop personnel, without catching the drain hole obstructions. The damage wasn't caught until the JSTARS was flown back to Robins and maintainers there conducted their own inspection.

Investigators found that human factors — particularly Northrop personnel failing to properly conduct the pre-flight inspections that should have caught the obstructed drain holes — contributed to the problems.

The water damage affecting that JSTARS was known as a Class A issue, meaning it cost at least $2 million to repair. The issues discovered with the other JSTARS were Class C mishaps, costing $50,000 to $500,000 to repair, or Class D mishaps requiring $20,000 to $50,000 to repair.

The incidents also raised concerns about the quality of work being done at Northrop's Lake Charles depot.

Officials at Northrop Grumman declined to discuss personnel matters.

The company "remains steadfast in its commitment to quality and safety to ensure the high-demand Joint STARS fleet is mission-ready for America's war fighters," Northrop spokesman Vic Beck said in a statement to Air Force Times. "Since this incident occurred in July 2016, we have implemented operational and process improvements in partnership with our Air Force customer."