(AP) — NASA still doesn’t know exactly how water gushed into the helmet and suit of a spacewalking astronaut.

Mission Control officials acknowledged Tuesday that Italian astronaut Luca Parmitano could have choked or even drowned from all the water that accumulated in his helmet.

His spacewalk was aborted soon after the problem cropped up, and he made it back safely inside with help from his spacewalking partner, American Christopher Cassidy.

NASA says between one and one-and-a-half liters leaked into Parmitano’s suit. Officials doubt his drinking water bag is to blame. The only other possibility is water from his garment cooling system.

The spacewalk lasted just 1½ hours and was one of the most harrowing in decades. Officials described Italy’s first spacewalker, as “grace under pressure.”