JOHNSON SPACE CENTRE, Texas, Feb 2: Nasa vowed on Sunday to “leave absolutely no stone unturned” in an exhaustive investigation into why the space shuttle Columbia broke up on re-entry into the Earth’s atmosphere, killing all seven astronauts on board in a fiery explosion.

As Americans mourned the deaths of seven of the “best and brightest” astronauts, police and soldiers fanned out across Texas in a grim and sometimes gory search for clues as to what caused the shuttle to break apart on Saturday, just 16 minutes from landing at its home base.

“We’re leaving nothing to chance. We’re looking at every piece of evidence, we’re securing all the debris and assuring we look at every possible angle of what could have caused this horrible accident,” Nasa administrator Sean O’Keefe told CBS’ “Face the Nation” programme.

“It’s been an accident of epic proportion.”

On ABC’s “This Week,” he said investigators would “leave absolutely no stone unturned in that process,” but that it was too early to speculate that insulation foam that came off in the launch and nicked a wing was behind the accident.

Columbia disintegrated high above the Texas plains almost 17 years to the day of the explosion that destroyed the space shuttle Challenger on Jan 28, 1986, also killing the six astronauts and one American school teacher on board.

Body parts, fragments and pieces of the shuttle Columbia were strewn across a vast area. In some rugged rural areas, horses and four-wheel drive vehicles were used in the search.

“We cannot avoid the obvious,” County Sheriff Thomas Kerss of Nacogdoches in Texas said on Sunday. “We have found remains.”

With the United States jumpy about a countdown to possible war with Iraq and with an Israeli astronaut as part of the crew, US government officials were quick to rule out terrorism in the disaster.

On “Fox News Sunday,” ambassador Mohammed Aldouri, Iraq’s representative to the United Nations, said only, “This is an unfortunate accident ... In Iraq they view this accident from that point of view. Certainly this is unfortunate for the families ... and for all people here in the United States.”

World leaders sent their condolences to the United States.

A grim-faced President George W. Bush, who led the nation in its grief and vowed the space shuttle programme would continue, attended church across the street from the White House where the congregation prayed for the astronauts.

BODY PARTS, WRECKAGE: Debris from Columbia rained down on fields, highways and a cemetery in northeast Texas, sending dozens of residents to hospitals for minor treatment after they were exposed to the smoldering metal wreckage.

No was directly injured by the catastrophic break-up on the ground. The destruction of what was America’s oldest space shuttle was heralded by an ear-splitting series of booms that rattled houses across the area.

Police and national guardsmen fanned out to guard pieces of wreckage ranging in size from a postage stamp to the trunk of a car, marking them with a traffic cone or yellow tape, until Nasa officials could collect them. The main search area is about 160-km long and 16-km wide.

Police outside Lufkin, Texas, were reported to have found human remains and in Nacogdoches a resident told Reuters he found human hair among debris in his yard.

The space agency has promised a seven-day-a-week, 24-hours-a-day probe. It will include studying everything from photos taken by spy satellites to examining every bit of debris recovered from the end of the 16-day scientific mission.

Nasa officials said the three remaining shuttles are grounded until the cause of the disaster is found and corrected and they are working to unlock the mystery of what happened.

Government agencies, including the National Transportation Safety Board and the Federal Emergency Management Agency, joined the investigation and the military was called in to help the search-and-recovery effort.

Keeping the shuttles grounded raised questions about the resupply of the International Space Station because the shuttle is the main resupply vehicle for the space outpost.

RUSSIAN LAUNCH: The world’s other big space power, Russia, sent condolences to Washington, but on Sunday went ahead with the launch of a cargo rocket carrying food and fuel to the space station.

Dramatic television images of the shuttle’s descent showed several white trails, tinged with fire, streaking through blue skies after the shuttle fell apart 16 minutes from home.

Speculation on the cause of the disaster centered on the loss of temperature sensors in the spacecraft’s left wing on re-entry.

The area had been hit by a piece of foam rubber insulation just after the shuttle’s launch and officials said they would look closely at the impact of the insulation.—Reuters