SpaceX and NASA say they finally know what caused its Crew Dragon capsule to explode during an April test of the spacecraft’s thruster system.

The culprit in the April 20 explosion was a leaky valve, which allowed some propellant to leak into high-pressure helium tubes around 100 seconds before the capsule’s thrusters were due to ignite, SpaceX said on Monday.

A slug of the propellant, known as nitrogen tetroxide (NTO), ended up in a high-pressure system, SpaceX wrote — resulting in an explosion. SpaceX said it plans to replace all of these valves with burst disks, which it says will “mitigate the risk entirely.”

SpaceX was joined by NASA, the Federal Aviation Administration (FAA), and the National Transportation Safety Board (NTSB) to investigate the explosion over the course of several months. The ground test at Cape Canaveral, Florida initially went well, with the craft successfully firing its Draco thrusters without any issues. The explosion took place as the SpaceX team was getting ready to fire the capsule’s more powerful SuperDraco thrusters.

NASA and SpaceX officials said at a press conference that the investigation was helped by the explosion taking place on the ground, which gave investigators the ability to search a relatively small area for evidence.

“We learned a very valuable lesson on something going forward, one that makes the Crew Dragon a safer vehicle,” said Hans Koenigsman, SpaceX’s vice president of build and flight reliability, according to The Verge.

The Crew Dragon is meant to ferry astronauts between the surface and the International Space Station (ISS). The company had initially planned to start those trips but the end of the summer, but it’s increasingly looking like crewed trips will be delayed until next year.

“My emphasis is really on making sure this is safe,” Koenigsman said. “So, end of the year, I don’t think it’s impossible, but it’s getting increasingly difficult.”

Despite the April explosion, SpaceX has recorded a number of successes as it moves to revolutionize commercial spaceflight. In June, it pulled off a nighttime launch of its Falcon Heavy Rocket, a feat that CEO Elon Musk called SpaceX’s “most difficult launch ever.”