The one-inch, gold plated pin is the chief suspect, but the space agency is continuing its investigation to be certain nothing else was to blame, said Bill Gerstenmaier, a senior shuttle manager.
The shuttle, commanded by Air Force Col. Eileen Collins, carrying five astronauts along with the world's most powerful X-ray telescope, leaked hydrogen fuel all the way into orbit.
A larger leak could have caused the damaged engine to shut down and forced NASA's first female commander to attempt an unprecedented emergency landing. As it was, the engines shut down a split-second before they were supposed to, leaving Columbia seven miles lower than its intended orbit.
Engineers believe the force of ignition a few seconds before liftoff knocked the pin from a tube and down through the combustion chamber of the right main engine. The pin--which was serving as a plug in a damaged oxygen-injection line--likely slammed against the nozzle.
The pin would not have created the three small holes found in three hydrogen-circulating tubes inside the nozzle. Rather, it would have weakened the stainless steel lines so much that they could no longer hold pressure and consequently ruptured, Gerstenmaier said.
The holes were discovered in the right engine nozzle immediately after Columbia landed Tuesday night. Gerstenmaier said the damage -- if it was, indeed, caused by the pin -- was on "the high side" for such a small object.
Columbia's right engine had two of these pins jammed into oxygen-injection tubes as plugs to keep the liquid oxygen from entering the damaged lines. One pin held, one didn't.
Such a rudimentary repair is not uncommon in older shuttle engines like this one. It's the first time, however, that a plug came loose during flight, said George Hopson, head of NASA's shuttle main engine program.
Gerstenmaier said it's too soon to know whether the problem will delay the next space shuttle flight, a radar-mapping mission by Endeavour in September. "We really need some more time to look at this and see what the implications are," he said.
The broken nozzle tubes have been sent for analysis to Rocketdyne in Canoga Park, Calif., where the engines are made.
Technicians, meanwhile, will search the launch pad in an attempt to find the missing pin. It may well be in pieces, shattered by the impact. "It's like looking for the proverbial needle in a haystack," Gerstenmaier said.
NASA's oldest space shuttle also sustained a momentary short circuit five seconds after liftoff, which knocked out the prime computers for two of the three main engines. Technicians hope to locate the electrical short in the next few days.