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A propellant leak caused the destruction of a SpaceX vehicle during an April test as it prepared to carry astronauts to the International Space Station.

Engineers behind the spacecraft investigated what happened alongside NASA, the US Air Force and the National Transportation Safety Board. Now, SpaceX and NASA say they are nearly certain of the cause of the anomaly, releasing the results of the investigation in a teleconference with reporters.

SpaceX engineering leader Hans Koenigsmann said it was now “increasingly difficult” for SpaceX to expect a crewed launch of its new spacecraft in 2019, another delay for an operation originally planned for 2017.

“In a lot of ways, this was a gift for us,” Kathy Lueders, NASA’s manager for the commercial crew program, said. “It was a test on the ground, we had a lot of instrumentation on the vehicle, we had high speed cameras, we were able to get the hardware and the data…through this process, we will continue to learn things that will help us fly safer.”

The Crew Dragon, which had just returned from a successful dress rehearsal flight to the ISS, was totally destroyed in an explosion, while being prepped for a ground engine test. Smoke pouring into the sky at Cape Canaveral and a leaked video of the conflagration alerted the public, but no one was injured thanks to routine safety precautions taken by SpaceX and NASA.

According to Koenigsmann, engineers prepared the Dragon’s escape engines for ignition by raising the pressure in the propellant system. About a cup of liquid oxidizer had leaked into the wrong plumbing, and when the pressure was turned on, that slug of liquid oxidizer impacted a titanium valve. Upon contact, the oxidizer and titanium reacted explosively.

Koenigsmann said that going forward, SpaceX engineers would isolate the oxidizer from the pressurization system and replace the four titanium valves with a simpler component called a “burst disc.” He said the impact on the schedule might be “relatively minor,” but said about 20% of the accident investigation remained to be done and that other documented issues needed to be solved as well.

Elon Musk’s space company has been working since 2011 to develop a new vehicle, which would replace the space shuttle, for ferrying astronauts to the International Space Station. Musk says human spaceflight is his company’s number one goal. But further delays may test the government’s patience with private companies providing transportation as a service.

Before the accident, SpaceX had hoped to perform its first crewed mission this year. The unexpected explosion took place ahead of a demonstration to show that the Dragon’s rocket thrusters can carry it to safety if the larger rocket carrying it into orbit malfunctions. Called an abort test, it is one of the final milestones the Dragon needs to accomplish before going into service.

The Dragon spacecraft is designed to carry as many as seven people to the International Space Station. The astronauts who will fly on Dragon are trained test pilots, have been closely involved with its design and test process. They’ve always expressed confidence that when SpaceX and NASA engineers sign off on the engineering of the vehicle, they’ll be ready to fly on it.

NASA’s other commercial crew contractor is aerospace giant Boeing. The company’s vehicle, called Starliner, needs to complete its own abort test and dry-run to the ISS, both of which NASA officials now expect to take place sometime this summer.

NASA has announced that once the spacecraft are up and running, they can be used to ferry paying individuals to the ISS for tourist visits or to perform corporate research and development in the orbital outpost.