SpaceX is still homing in on why its Falcon 9 rocket exploded after liftoff last week, unable to resolve conflicting data radioed back to the ground before the explosion, CEO Elon Musk said on Tuesday.

“It’s a huge blow to SpaceX. We take these missions incredibly seriously,” Musk said at a webcast International Space Station research and development conference in Boston.

So far, no one theory can account for the data collected prior to the rocket’s breakup about 2-1/2 minutes after launch from Cape Canaveral Air Force Station on June 28.

“The data does seem to be quite difficult to interpret. Whatever happened is not a simple straightforward thing,” Musk said.

SpaceX plans to take its findings to the Federal Aviation Administration, which oversees U.S. commercial launches, NASA and some customers to see if an outside eye can help resolve the conundrum.

“We want to see if we can get to what the most likely root cause is, look at both what we think most likely happened, and then anything that’s a close call and try to address all of those things and maximize probability of success for future missions,” Musk said.

Musk declined to elaborate on the most likely cause of the accident, but expects to be able to release more information by the end of the week.

“At this point, the only thing that’s really clear is there was some kind of overpressure event in the upper-stage liquid oxygen tank. The exact cause and the sequence of events, there is still no clear theory that fits with all the data,” Musk said.

SpaceX is one of two commercial companies hired by NASA after the shuttles were retired to fly cargo to the station, a $100 billion research laboratory that flies about 260 miles (418 km) above Earth.

The other U.S. cargo line, operated by Orbital ATK, has been grounded since October following another launch accident.

Russia lost a cargo ship after a botched launch in April, but successfully delivered a replacement on Sunday. The accidents had no immediate impacts on the station, which is staffed by rotating crews of astronauts and cosmonauts.

SpaceX has a backlog of nearly 50 Falcon rocket launches on its schedule, worth more than $7 billion. The company said after the accident it hopes to be back flying within the next several months.

(Editing by W Simon)