Space X's Falcon 9 rocket as it lifts off from space launch complex 40 at Cape Canaveral, Florida on June 28, 2015

The SpaceX Falcon 9 explosion was caused by a failed strut that allowed a helium bottle to burst free inside the rocket's liquid oxygen tank, CEO Elon Musk said Monday.

"One of those struts broke free during flight," Musk told reporters on a conference call to discuss the June 28 blast on what was supposed to be a routine cargo mission to the International Space Station.

"So the helium bottle would have shot to the top of the tank at high speed," said Musk, noting that the findings are the preliminary results of a weeks-long investigation by the California-based company.

The accident caused NASA to lose $110 million in equipment bound for the astronauts living in orbit, a US space agency spokesman told Congress earlier this month.

The struts in question are each two feet (60 centimeters) long and an inch (2.5 cm) thick, and were furnished to SpaceX by an outside supplier.

"We are not going to use these particular struts in the future," Musk said, adding that the company plans to begin individually testing each strut ahead of future launches.

Musk said SpaceX would return to flight with the Falcon 9 "no sooner than September."

He also said the problem is not expected to delay the company's goal of sending astronauts to space aboard its Dragon spaceship within the next two years.

Musk, the billionaire cofounder of PayPal who also heads Tesla Motors, said that SpaceX had had a seven-year record of safety in flight until the accident happened.

This June 28, 2015 grab from NASA TV shows the SpaceX Falcon 9 rocket with the unmanned Dragon cargo capsule on board exploding shortly after launching from Cape Canaveral, Florida

He admitted that SpaceX may have become "a little bit complacent," and that the blast offered an "important lesson" for the future.

The explosion happened just over two minutes after the rocket blasted off from Cape Canaveral, Florida.

The rocket failure was the third in a series of cargo disasters in the past eight months.

In October, US company Orbital's Antares rocket exploded after launch from Virginia, and in April, Russia lost contact with its Progress cargo ship shortly after liftoff.

© 2015 AFP