A NASA committee has again raised serious concerns about SpaceX’s rocket fueling procedure, months after a violent explosion at the Cape Canaveral Air Force Station in Florida.

The group had issued warnings in December 2015, before the September incident, but on Monday, it reiterated those fears that humans could be put in harm’s way during SpaceX launches.

The Falcon 9’s supercooled fueling system requires that crew board an attached space capsule before fueling begins, meaning they could be victims of any explosions. Boarding and then fueling is not common practice in the aerospace industry.

A SpaceX representative said after the meeting that the company “has designed a reliable fueling and launch process that minimizes the duration and number of personnel exposed to the hazards of launching a rocket.”

The company’s rockets have an emergency system in place where crew members can remove themselves from the situation at a moment’s notice. But the fuel means the rocket has to lift off around half an hour after fueling, whereas normally rockets are fueled hours before the crew gets into position. The company’s practices would mean that a manned Mars mission may need astronauts already in position inside the rocket.

SpaceX said that the company’s investigation of the September Falcon 9 incident will help evaluate the safety controls in place ahead of rocket launches. The company said that NASA approved a set of safety controls in July. At the advisory committee in December, though, the group unanimously voted to reject the company’s fuel procedure. NASA has yet to formally respond to the group’s latest concerns.

A breach in the cryogenic helium system is one theory for what happened to the Falcon 9 rocket in Florida in September. The company has yet to confirm, but last week it was reported that the explosion may have been caused by a fueling error. If the incident was due to human error, it means SpaceX likely won’t need to take their rocket back to the drawing board and undergo a costly redesign.

Photos via USLaunchReport/YouTube