Nelson adds that he ask spoken with Boeing CEO Kelly Ortberg, who told Nelson that Boeing is committed to fixing the Starliner problems.

NASA Associate Administrator Jim Free said the decision to return Starliner uncrewed came from "uncertainty in the margins" including uncertainty in the physics of what was going on in Starliner's thrusters.

Boeing (not at briefing) statement on NASA's decision: “Boeing continues to focus, first and foremost, on the safety of the crew and spacecraft. We are executing the mission as determined by NASA, and we are preparing the spacecraft for a safe and successful uncrewed return.”

NASA commercial crew program manager Steve Stich: there was "too much uncertainty" in the performance of the Starliner thrusters. If they had a better model for thruster performance, would have taken different course of action.

NASA's Norm Knight says Wilmore and Williams support NASA's decision fully. He adds the agency has not decided which of the four Crew-9 members will be taken off the free up seats for Wilmore and Williams.

NASA's Ken Bowersox: thought we could get there (on returning Starliner with astronauts) until about a week ago. Adds polling among NASA personnel was unanimous for an uncrewed return.

Nelson says he is "100%" certain that Starliner will fly again with astronauts on board.

Stich and Bowersox said NASA has not decided if another Starliner crew test flight will be required. Will work with Boeing afterwards on path forward.

Bowersox: we don't know how much we can use the Starliner thrusters before we encounter a problem; that was key factor in decision to return uncrewed.