The U.S. Federal Aviation Administration (FAA) on Monday said SpaceX must investigate why the second stage of its Falcon 9 rocket malfunctioned after a NASA astronaut mission on Saturday, grounding the rocket for the third time in three months.

After SpaceX on Saturday launched two astronauts to the International Space Station (ISS) for NASA, the rocket body that had boosted the crew further into space failed to properly re-light its engine for its "deorbit burn," a routine procedure that discards the booster into the ocean after completing its flight.

The astronaut crew arrived at the ISS safely on Sunday as planned. The FAA said there were no injuries or property damage linked to the booster mishap.

The malfunction caused the booster to fall into a region of the Pacific Ocean outside of the designated safety zone that the FAA approved for the mission.

SpaceX said the booster "experienced an off-nominal deorbit burn. As a result, the second stage safely landed in the ocean, but outside of the targeted area."

"We will resume launching after we better understand root cause," SpaceX wrote in a post on X.

Saturday's mishap was the third to trigger an FAA grounding in the past three months. Before that, groundings were rare for Falcon 9, SpaceX's centerpiece rocket, which much of the Western world relies on for accessing space.

The rocket was grounded in July after a second-stage issue sent a batch of SpaceX-built Starlink satellites on an orbital path to destruction, marking SpaceX's first mission failure in more than seven years. SpaceX resumed Falcon 9 flights 15 days later.

In August, another grounding was triggered by the failure of a Falcon 9 first stage to land back on Earth, a mishap that did not affect mission success. The company returned to flight three days later.

SpaceX is likely to seek FAA approval to resume flights in a similar manner, while its engineering investigation continues with oversight by the FAA. The agency regulates rocket launches and rocket re-entries to the extent they may affect public safety.

SpaceX has launched an average of two to three rockets a week since the beginning of 2024. Falcon 9's first stage is reusable, but its second stage is not.

The grounding comes at a testy time for SpaceX and the FAA – the two have been feuding openly over the pace of launch licensing regulations and a pair of fines the FAA imposed on SpaceX for allegedly violating its Falcon launch licenses in 2023.

SpaceX has complained publicly that the FAA has been slow to approve the license for Starship's fifth flight test, which involves far more ambitious testing objectives than the previous flight.