The SpaceX explosion on June 28th was caused by a failed strut in the rocket's upper stage liquid oxygen tank, SpaceX chief executive officer Elon Musk said today. The strut was one of several hundred inside the tank used to hold down the helium pressure vessels, which help to pressurize the rocket. According to Musk, the strut was designed to handle 10,000 pounds of force, but failed at just 2,000 pounds of force.

"This is the best of what we know thus far."

"This is the best of what we know thus far," said Musk during a press conference. "We emphasize this is an initial assessment, and further investigation may reveal more over time."

The strut was a steel rod that’s about two feet long and one inch thick. Struts like the one that failed have flown on several previous Falcon 9 flights before. Musk said SpaceX still doesn’t know why the steel rod snapped, but it’s possible that its material was faulty. Musk didn’t name the strut's supplier, but did say it may be that one strut of thousands wasn’t up to code.

When the strut snapped, the helium tanks released a lot of helium into the upper-stage liquid oxygen tank, Musk said. That put too much pressure on the tank and caused it to explode.

The Dragon capsule, filled with supplies for the International Space Station, survived the explosion and continued to communicate data to ground control for a while afterward. If the Dragon had been equipped with different software, the capsule would have been able to deploy its parachutes and save its contents. Future versions of the Dragon cargo capsule will have this capability, Musk said.

"It's the first time we've had a failure in seven years so, to some degree, the company as a whole got a little complacent," said Musk. "Especially with all the successes in a row, I think this is an important lesson and something we'll take with us into the future."

"This is an important lesson and something we'll take with us into the future."

Nearly a month ago, SpaceX's Falcon 9 disintegrated during its ascent to the International Space Station. SpaceX has a contract with NASA to launch commercial resupply missions to the station; that rocket launch was the seventh of 12 planned missions. The Falcon 9 was carrying 4,000 pounds of supplies, including food and water for the station's crew and a new International Docking Adaptor. The IDA, meant to be mounted on the outside of the station, will allow future US crewed spacecrafts from commercial spaceflight companies to dock with the ISS. SpaceX and Boeing have contracts to ferry astronauts to the ISS beginning in 2017.

At the 2015 ISS Research and Development Conference, Musk described the failure as a "huge blow for SpaceX." Until the June incident, SpaceX had an impeccable launch record. This recent rocket loss was the first major failure out of 19 total Falcon 9 launches. As a result of the accident, SpaceX has postponed its launches of the Falcon 9 for the next couple of months. Their potential next launch, which was originally scheduled for August 9th, was meant to carry the National Oceanic and Atmospheric Administration's Jason-3 Earth observation satellite to orbit. However, it's unclear what SpaceX's next launch will be. Musk said the launch postponement will result in hundreds of millions of dollars in lost revenue.

Update, 4:30PM, July 20th: SpaceX will switch to different struts with even better safety records, according to Musk. The company will also change their procedures for testing the struts before lift off — SpaceX employees will conduct individual unit tests on every single steel beam. That shouldn’t raise the costs a "significant amount," he said.

Musk said the Falcon 9 failure shouldn’t affect the company’s contract with NASA for its Commercial Crew Program, which tasks SpaceX with ferrying astronauts to and from the ISS. However, the first test launch of SpaceX’s Falcon Heavy rocket, which will carry these astronauts into space, has been postponed. Originally scheduled for later this year, the first Falcon Heavy launch is now slated for spring of 2016.

Correction: The struts are used to anchor the helium pressure vessels inside the liquid oxygen fuel tank. A previous version of this story suggested they were used to hold the pressure vessels together.

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