NASA was last night accused of ignoring warnings from its own technicians and government auditors that a space shuttle disaster was imminent.

As the space agency named a retired Navy admiral to head an independent investigation into Saturday’s Columbia disaster, calls to scrap the shuttle programme were increasing.

It emerged that as recently as Thursday a congressional audit report had expressed concern about monitoring of shuttle safety.

In April last year Nasa’s own safety advisers, the Aerospace Safety Advisory Panel, said that the shuttle programme raised “the strongest safety concerns” it had voiced in 15 years. That followed a report in 2000 by the General Accounting Office, Congress’s investigative arm, that said workforce reductions were “jeopardising Nasa’s ability to safely support the shuttle’s planned flight rate”.

The biggest search and recovery operation in US history was taking place across 6,400 square miles of eastern Texas and Louisiana, where thousands of fragments of the space shuttle remained strewn yesterday.

Columbia, the oldest and heaviest of Nasa’s four shuttles, disintegrated 39 miles above the Texas plains as it re-entered the Earth’s atmosphere for a landing in Florida, killing all seven astronauts on board, almost 17 years to the day of the explosion that destroyed the shuttle Challenger in January 1986.

At two sites in eastern Texas charred human remains were recovered yesterday, including a torso, thigh bone and skull on a rural road, in addition to a space helmet. Police and national guardsmen fanned out to guard pieces of wreckage ranging in size from a postage stamp to the bed of a pick-up truck. The main “debris belt” was about 100 miles long and ten miles wide.

Ron Dittemore, the shuttle programme director, said last night that problems with Columbia were first detected at 7.53am on Saturday, five minutes before contact was lost.

At 7.54am, he said, mission control noticed an “unusual temperature rise” on the left hand side of the spacecraft.

At 7.58 there was an “increased drag” on the left side, indicating “a rough or missing tile”. The flight control system tried to counter the leftward tilt by bringing Columbia rightwards.

Shortly afterwards contact with Columbia was lost, although there was another 32 seconds of data produced by the shuttle’s monitors that was available for analysis.

There was no evidence that the astronauts were alarmed. He emphasised that a tile knocked off the left wing during take-off was not the only focus of the investigation. “We believe the loss of a tile would not represent the loss of a vehicle,” he said, adding that Nasa was also looking at the structure of the craft and its flight control.

Emphasising that terrorism was not a factor in the disaster, Nasa named Harold Gehman, the retired admiral who helped to investigate the USS Cole bombing in Yemen harbour, to head a special commission into the Columbia’s fate.

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