"Columbia investigators said Tuesday they are growing more certain of what brought down the shuttle: A seal on the left wing was struck by foam during liftoff and fell off the next day, creating a gap that let in enough scorching gases during re-entry to rip the ship apart.
A seal from Columbia's left wing is now believed to be the mystery object that floated away in orbit and it was almost certainly struck by something like a chunk of foam before it came off, the accident investigators said."
http://abcnews.go.com/wire/US/ap20030422_2561.html