The US space agency labeled the 2024 test flight of the Starliner capsule a "Type A" mishap -- the same classification of the deadly Challenger and Columbia shuttle disasters -- a category that reflects the "potential for a significant mishap," it said.

The failures left a pair of NASA astronauts stranded aboard the International Space Station for nine months in a mission that captured global attention and became a political flashpoint.

"Starliner has design and engineering deficiencies that must be corrected, but the most troubling failure revealed by this investigation is not hardware. It's decision-making and leadership," said NASA administrator Jared Isaacman in a briefing.

"If left unchecked," he said, this mismanagement "could create a culture incompatible with human spaceflight."

The top space official said the investigation found that a concern for the reputation of Boeing's Starliner clouded an earlier internal probe into the incident.

"Programmatic advocacy exceeded reasonable bounds and place the mission, the crew and America's space program at risk in ways that were not fully understood at the time," Isaacman said.

He said Starliner currently "is less reliable for crew survival than other crewed vehicles" and that "NASA will not fly another crew on Starliner until technical causes are understood and corrected" and a problematic propulsion system is fixed.

But the commissioner insisted that "NASA will continue to work with Boeing, as we do all of our partners that are undertaking test flights."

Isaacman also had harsh words for internal conduct at NASA.

"We managed the contract. We accepted the vehicle, we launched the crew to space. We made decisions from docking through post-mission actions," he told journalists.

"A considerable portion of the responsibility and accountability rests here."

- 'We failed them' -

In June 2024 Butch Wilmore and Suni Williams embarked on what was meant to be an eight-to-14-day mission. But this turned into nine months after propulsion problems emerged in orbit and the Starliner spacecraft was deemed unfit to fly them back.

The ex-Navy pilots were reassigned to the NASA-SpaceX Crew-9 mission. A Dragon spacecraft flew to the ISS that September with a team of two, rather than the usual four, to make room for the stranded pair.

The duo, both now retired, were finally able to arrive home safely in March 2025.

"They have so much grace, and they're so competent, the two of them, and we failed them," NASA associate administrator Amit Kshatriya told Thursday's briefing.

"The agency failed them."

Kshatriya said the details of the report were "hard to hear" but that "transparency" was the only path forward.

"This is not about pointing fingers," he said. "It's about making sure that we are holding each other accountable."

Both Boeing and SpaceX were commissioned to handle missions to the ISS more than a decade ago.