NASA is starting to get really concerned about SpaceX, and officials are starting to question whether the Elon Musk-owned company is putting astronauts at risk after a Sept. 1 explosion on a Florida launch pad. A NAsA advisory committee has written a letter expressing worry about the way SpaceX fuels its rockets, doing so with astronauts aboard.

SpaceX plans to eventually take astronauts to the International Space Station aboard its Crew Dragon spacecraft, but NASA officials don’t like the fact that the company fuels its spacecraft while astronauts are on board. SpaceX does this because it is using a unique method of adding chilled liquid oxygen to fit more fuel in the tank and enable a higher weight, but this must be done immediately before launch so the astronauts would have to be already on board.

While SpaceX has assured NASA that it knows what it’s doing, NASA isn’t so sure, especially after a rocket exploded on the launch pad a couple months ago.

“There will be continued work ahead to show that all of these controls are in place for crewed operations and that the verifications meet NASA requirements,” SpaceX said in a statement. “These analyses and controls will be carefully evaluated in light of all data and corrective actions resulting from the anomaly investigation.”

The latest update on the anomaly, dated Oct. 28, indicates that SpaceX believes it has narrowed down the problem to a system in the second stage liquid oxygen tank.

“The Accident Investigation Team continues to make progress in examining the anomaly on September 1 that led to the loss of a Falcon 9 and its payload at Launch Complex 40 (LC-40), Cape Canaveral Air Force Station, Florida,” the update reads. “Since the incident, investigators from SpaceX, the FAA, NASA, the US Air Force and industry experts have been working methodically through an extensive fault tree to investigate all plausible causes. As part of this, we have conducted tests at our facility in McGregor, Texas, attempting to replicate as closely as possible the conditions that may have led to the mishap.

“The investigation team has made significant progress on the fault tree,” it adds. “Previously, we announced the investigation was focusing on a breach in the cryogenic helium system of the second stage liquid oxygen tank. The root cause of the breach has not yet been confirmed, but attention has continued to narrow to one of the three composite overwrapped pressure vessels (COPVs) inside the LOX tank. Through extensive testing in Texas, SpaceX has shown that it can re-create a COPV failure entirely through helium loading conditions. These conditions are mainly affected by the temperature and pressure of the helium being loaded.”