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By Associated Press

SpaceX still is trying to figure out what caused its rocket to break apart during liftoff nine days ago, but it's getting close to an answer, the company's chief executive said Tuesday.

The unmanned Falcon 9 rocket carrying cargo for the International Space Station had just lifted off on June 28 when the accident occurred. Speaking Tuesday at a conference in Boston, SpaceX founder Elon Musk said the trouble appeared to be in the upper stage, with an over-pressurization of the liquid oxygen tank. Nevertheless, the California-based company is putting together what he calls a "super-detailed" timeline, millisecond by millisecond.

So far, a leading theory has yet to emerge, Musk told attendees of the space station research and development conference. The available data are difficult to interpret, and so "whatever happened is clearly not a sort of simple, straightforward thing," he said. Nevertheless, he hopes to have something definitive to say by week's end.

"Obviously, it's a huge blow to SpaceX. We take these missions incredibly seriously," Musk said. The accident occurred on Musk's 44th birthday — "a real downer — definitely a low point."

The main intent was to deliver more than 5,000 pounds of supplies and equipment to the space station, including the first of two docking ports for crew capsules in development by SpaceX and Boeing. The Dragon capsule carrying all the cargo slammed into the ocean, along with the rocket wreckage.

It was the third lost shipment in eight months — Orbital Sciences Corp. in October, the Russian space program in April, then SpaceX in June. NASA's space station program manager Mike Suffredini said at the conference that the three failed missions have had "a big impact to us."

"Never in my wildest dream," Suffredini said, did he think so many cargo ships would go down in such a short period. The same crew provisions were reflown to make up for the failed missions, and three times ended up being lost. "We'll keep trying until we get that right, because the crew does need their clothes," Suffredini noted wryly.

On a brighter note, a Russian cargo ship made a successful delivery on Sunday. Japan's space agency is on track to send up more supplies in mid-August. And Orbital Sciences plans to fly its Cygnus cargo craft on United Launch Alliance's Atlas rocket later this year while it overhauls its own Antares rocket.

Suffredini pointed out that the three spacefliers aboard the space station are healthy and safe, with plenty of scientific work. Three more crew members are set to launch from Kazakhstan later this month.