CAPE CANAVERAL, Fla. — SpaceX says a leaky valve caused its crew capsule to explode during a test back in April.

The company announced the preliminary results of its accident investigation Monday.

SpaceX's crew Dragon capsule had just returned from a successful test flight to the International Space Station and was undergoing a ground test at Cape Canaveral, Florida. The abort-system thrusters were milliseconds away from firing, when the spacecraft exploded and was destroyed.

Hans Koenigsmann, a SpaceX vice president, says a check valve allowed liquid oxidizer to leak into the high-pressure abort system. The titanium valve ignited, leading to the blast. He says the check valves will be replaced with more reliable disks.

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Koenigsmann says while it's not impossible, it's becoming "increasingly difficult" to fly astronauts on the capsule by year's end.

SpaceX Crew Capsule
In this Dec. 18, 2018 photo provided by SpaceX, SpaceX's Crew Dragon spacecraft and Falcon 9 rocket are positioned inside the company's hangar at Launch Complex 39A at NASA's Kennedy Space Center in Florida, ahead of the Demo-1 unmanned flight test. (Space X via AP)