The bottom half of the faulty rocket joint that caused the space shuttle disaster has been retrieved from the ocean bottom, NASA announced today. The space agency said part of the piece's upper edge had been burned away.

With recovery of the piece, a 10-by-6-foot section of steel rocket casing that weighs a ton, investigators have the matching upper and lower portions of the joint. Their retrieval was the top priority of the massive salvage effort mounted to recover debris of Challenger, and the National Aeronautics and Space Administration today suspended further efforts to recover booster debris.

Together, the matching pieces provide physical evidence to support the conclusion reached earlier through photography and other data: the joint caused the shuttle to break apart.

The shuttle's two solid-rocket boosters -- cylinders 149 feet tall and 12 feet in diameter -- come in four sections each, and it is the joint between the two lower sections of the right booster that began to spew flames and hot gases shortly after Challenger's launch.

The leak in the joint cut the booster's bottom attachment loose from the shuttle, allowing the booster to pivot into the top of the external fuel tank, ripping it and the orbiter apart in a fireball Jan. 28.

The joint is designed to form an airtight seal using a combination of rubber O-rings and putty. Investigators think it failed because of a number of factors, including design problems and below-freezing weather the night before the launch.

The joint had a five-year history of performance problems before the Challenger accident that investigators say NASA knew were serious but failed to correct.