Elon Musk’s SpaceX, fresh off a series of big wins with its space program, is looking to make the Falcon 9 rocket more powerful by keeping propellant at very cold temperatures to shrink the amount of space it takes up. But a NASA advisory group said in a letter that has resurfaced that the method is potentially unsafe and could put lives at risk.

In a new report, the Washington Post is bringing up concerns raised in a 2015 letter from an advisory group, which is timely since Elon Musk and SpaceX are pushing forward with a plan that has had concerns for years. Safety experts warn that because the propellant is kept at extreme temperatures, it would need to be loaded just before takeoff while astronauts are aboard, raising the risk of an accident that could set off a catastrophic and deadly explosion. The letter comes at a time when SpaceX is getting ready to launch human sinto space.

Their concerns have some resemblance to reality, as in September 2016 a SpaceX Falcon 9 rocket blew up while being fueled for an engine test. Because no one was on board, no one was hurt, but had it happened with astronauts aboard there would have certainly been loss of life. The explosion obliterated the multimillion-dollar satellite it was meant to carry into orbit.

“We are concerned that there may be insufficient precooling of the tank and plumbing with the current planned oxidizer fill scenario, and without recirculation there may be stratification of oxidizer temperature that will cause a variation in the input conditions to the oxidizer pump,” reads the the NASA ISS Advisory Committee letter, dated Dec. 9, 2015. “In summary, we are deeply concerned about introducing the practice of fueling with the crew onboard, and about the lack of even a recirculation pump for oxidizer conditioning on Falcon 9.”