Here are significant developments in the investigation by the Columbia Accident Investigation Board:

Feb. 1: Just hours after the Columbia accident, shuttle program manager Ron Dittemore says NASA will study whether a piece of foam insulation that broke off the fuel tank and hit the left wing during liftoff might be the cause. He urges caution: "There are a lot of things in this business that look like the smoking gun but turn out not even to be close."

Also, retired Navy Adm. Harold L. Gehman Jr. is asked to lead a panel investigating the cause. Seven members are named the next day; ultimately the board grows to 13.

Feb. 5: After days of analysis, NASA backs away from the theory that a piece of foam might be the root cause of the space shuttle's disintegration over Texas. "It just does not make sense to us that a piece of debris would be the root cause," Dittemore says. "There's got to be another reason."

Feb. 13: The Columbia Accident Investigation Board says evidence indicates something caused a hole that let superheated re-entry gases into the shuttle's left wing. The board didn't speculate on whether foam was the culprit, but says every theory is on the table.

Feb. 26: E-mails released by NASA reveal that one day before the disaster engineers worried the shuttle's left wing might be so damaged it could burn off. The warnings were never sent to NASA's top officials. The e-mails also revealed that during Columbia's 16-day mission a request was made, then withdrawn, for satellite images to check for damage to the shuttle.

Feb. 28: NASA releases a video showing the final minutes of the Columbia astronauts' lives, as the shuttle descends through the atmosphere.

March 6: In the first public hearing of the investigation board, NASA officials defend decisions that gave private contractors much of the direct responsibility for operating the space shuttle.

March 21: Newly released documents show that NASA had flagged as a major concern the recurring shedding of foam insulation on the shuttle's external fuel tank. But no action was taken to prevent it.

March 28: The military agrees to a NASA request to use spy satellites to photograph future shuttles in space as a way to spot potential problems.

March 31: E-mail records show Rodney Rocha, NASA's chief shuttle engineer, wrote -- but never sent -- a warning that failure to seek photographs of possible damage to the shuttle's left wing was wrong and "bordering on irresponsible."

April 8: Board members say NASA relied on a flawed analysis of debris damage and allowed a web of miscommunication to block engineers from getting photos of the shuttle in orbit.

April 17: The Columbia investigation board urges NASA to more thoroughly inspect space shuttle wings and require in-orbit picture-taking.

April 23: Shuttle program manager Ron Dittemore says he is resigning after the Columbia investigation concludes.

May 23: Gehman says NASA could have launched another shuttle in a risky mission to rescue the Columbia astronauts if it had realized the severity of the wing damage early on.

June 24: The investigation panel says a suitcase-sized chunk of foam that smashed into the left wing is "the most probable cause."

June 30: NASA documents reveal that even as NASA engineers debated possible damage to the shuttle, a flight director e-mailed the astronauts to say there was "absolutely no concern" that breakaway foam harmed the spacecraft.

July 7: A chunk of foam insulation fired at shuttle wing parts blows open a gaping 16-inch hole. "We have found the smoking gun," says Columbia accident investigator Scott Hubbard. It was the seventh foam impact test on a mock-up of a shuttle wing.

July 22: Columbia's top mission manager, Linda Ham, says she didn't seek satellite pictures of the spaceship which some engineers wanted because no one expressed any fears of serious damage to the craft. She also says she didn't know exactly who was making the photo request.

July 30: The board urges NASA to provide better, quicker pictures of separating shuttle fuel tanks and the vulnerable underside of the spaceships' wings to check for damage from launch debris.