Washington ? A flaw in the system of explosive bolts that connect major components of the space shuttle had the potential to cause “catastrophic” damage and almost certainly will need to be fixed before the three grounded shuttles can fly again, investigators said Thursday.

Radar images reviewed by the accident team show that a piece of debris — possibly a 40-pound chunk of an explosive bolt or the 11-pound container designed to capture it — flew off Columbia just more than two minutes after liftoff.

The discovery, disclosed at a Thursday briefing, means there may have been at least two kinds of debris that menaced the shuttle during the Jan. 16 launch.

Investigators already knew that a chunk of foam insulation came loose from the shuttle’s external fuel tank 81 seconds after liftoff and, as shown in video of the launch, struck the left wing.

The Columbia Accident Investigation Board has adopted a working hypothesis, supported by recent testing, that the foam could have done enough damage to cause a breach in the wing that led to the Feb. 1 destruction of shuttle and crew.

Impact tests conducted last week in San Antonio showed that the foam insulation could crack the shuttle’s leading-edge heat shielding. In addition to two cracks reported immediately after the June 6 test, further analysis has revealed a third crack, board member Scott Hubbard reported Thursday.

“We are not changing our working scenario,” said Maj. Gen. John Barry, another member of the board. “However, we need to close (the explosive-bolts issue) out, and we need to make sure we understand it … certainly for the future.”