U.S. space agency NASA has publicly revealed for the first time ever that it was because of a design error that the launch attempt of SpaceX failed in 2015.

According to a summary report published here, NASA engineers have said that design error in the Falcon 9 rocket at the time was the initiating factor for the destruction of a rocket that was to be launched to International Space Station in 2015.

SpaceX had already offered an explanation at the time for the failure of its CRS-7 mission. The rocket company had passed the buck on a subcontractor that manufactured a steel strut which snapped during the flight, allowing a helium bottle to break loose and crash through a liquid oxygen tank, which burst, combusted and destroyed the vehicle. NASA says SpaceX should known that the strut was not suited for spaceflight, calling the decision to use it a “design error.”

Investigators of the SpaceX launch failure have also noted that all the issues that were identified during the investigation have been resolved by SpaceX quickly before launch of January 2016. Their inquiry was completed in December 2015; NASA said it released the summary more than two years later “to maintain historical data of the mishap.”

SpaceX no longer makes this version of the Falcon 9, and has flown their rocket successfully 31 times since the accident, with one rocket destroyed during a fueling mishap in September 2016.

There were rumors that NASA wasn’t in agreement with SpaceX’s assessment of the launch failure, and a 2016 Inspector General report referenced possible additional causes, like poor installation practices, that could be behind its failure.

At the time, the US space agency had demanded a large reorganization at SpaceX to adopt more reliability and safety controls in its hardware supply chain to ensure that accidents like this don’t happen again. SpaceX also ate the cost of the failure, and re-jiggered its program to provide additional flights and services to NASA.