US investigators have recovered a large piece of one of the space shuttle Columbia's wings which could provide important clues to the cause of the disaster.

There has been a significant recovery... we do have a large piece of one of the wings Michael Kostelnik Nasa

Investigators are working to establish which wing the piece belonged to.

Nasa appeared to play down a fresh piece of photographic evidence which reportedly shows serious structural damage to the shuttle's left wing

Mourning for the seven-strong crew of Columbia, which disintegrated as it returned to Earth last Saturday, has continued with a service at Kennedy Space Center in Cape Canaveral, Florida.

The wing fragment recovered from the Fort Worth, Texas, area features:

• 66 to 68.5 centimetres (26 to 27 inches) of carbon-composite panel which reinforces the leading edges of space shuttle wings for thermal protection during the searing heat of atmospheric re-entry

• 46 cm (18 inches) of actual wing structure

Mysterious 'streak'

Nasa is also studying a close-up image taken by a powerful ground-based US Air Force camera in New Mexico as the shuttle re-entered the atmosphere.

You cannot tell from that photograph that an event occurred Ron Dittemore

But the agency said on Friday that it was too early to assess its worth.

The fuzzy, batwing-shaped silhouette of the shuttle appears to show a dark grey streak behind the left wing.

However, shuttle programme manager Ron Dittemore said it would take further study to determine whether the image showed a problem with the shuttle, and if the streak was from Columbia or only a technical aberration in the photograph.

"It is not clear to me that there is something there," he said.

SHUTTLE BREAK-UP

Re-entered atmosphere at 12,500 mph (20,000 km/h) Disintegrated 40 miles (65 kilometres) above the Earth Debris scattered over Texas and Louisiana - reports now being checked of sightings in California and Arizona

Click here for a step-by-step guide to the disaster

Earlier, a specialist journal, Aviation Week & Space Technology, suggested that photos from the telescope showed a jagged area on the wing, including areas protected by reinforced material intended to counteract extreme heat.

Mr Dittemore said that there did appear from the photo to be "something just a little different about the left-hand side behind the wing than the right-hand side" and it was an "area of investigation".

But he stressed that the picture did not reveal the cause of the disaster.

"It does not indicate whether an event occurred on launch day, in orbit or even during re-entry," he said.

"You cannot tell from that photograph that an event occurred."

'No scapegoats'

Nasa's administrator, Sean O'Keefe, has sought to reassure the agency's employees that the investigation into the disaster will be as fair as possible.

Meeting members of the independent advisory board leading the inquiry, he said:

"Right now, we want to find out what happened. No one person will be fingered."

Earlier, Mr O'Keefe was among other Nasa workers attending the memorial service on Kennedy Space Center's runway where Columbia had been due to land.

Nasa jets flew in "Missing Man" formation as fellow astronauts such as Robert Crippen, Columbia's first pilot, paid their respects.

The names of all seven of the dead are to be engraved on the Space Mirror Memorial at Cape Canaveral.

The remains of Ilan Ramon - the first Israeli in space - are to be repatriated for burial. There are no details for funerals for the six American astronauts, who included Indian-born Kalpana Chawla.