SpaceX may be grounded for a little while. The Government Accountability Office (GAO) has raised concerns about the safety of private aerospace manufacturer’s Falcon 9 rockets, noting they suffer from persistent cracking according to a Wall Street Journal report.

According to the preliminary findings from an investigation into SpaceX and rival company Boeing, the rockets from the company founded by Elon Musk may not be fit for future manned missions.

The issue stems from a pattern of problems with turbine blades that deliver fuel into the rocket engines. The blades are prone to cracking, which is considered a major threat to the safety of the craft and will require a redesign of the Falcon 9’s turbopumps.

SpaceX said its rockets are designed to withstand such cracking without affecting its performance, but will be updating the design to avoid the issue entirely. The National Aeronautics and Space Administration has reportedly told SpaceX the cracks are an unacceptable risk for manned flights.

When the GAO report is finalized and published in the coming weeks, it will mark the first public disclosure of a serious defect in SpaceX’s Falcon 9 rockets. However, the government has reportedly known of the issue for years, and the cracks had been found in rockets launched as recently as September 2016.

The news marks a continuation of what has been a rough couple of months for Musk’s aerospace company. In September 2016, SpaceX had its first major setback when one of its Falcon 9 rockets exploded on the launchpad in Cape Canaveral, Fla.

While SpaceX called the incident an anomaly, it caused considerable damage—destroying an AMOS-6 communications satellite from Israeli company Spacecom that was to be part of an effort from Facebook to deliver internet connectivity to parts of Africa. The explosion was reported to have scared off other partners looking to launch with SpaceX.

Investigations into the issue also caused delays to future launches from the company. A launch planned for December 2016 was pushed into January, then postponed again due to uncooperative weather conditions.

The GAO has reportedly determined neither SpaceX nor competitor Boeing will be able to meet their goals of launching missions to ferry astronauts to the International Space Station by 2018.