Search for debris and human remains from the Columbia shuttle continued on Sunday even as investigators struggled to determine the cause of one of the worst space mishaps that sent shock waves across the world.

Indian American astronaut Kalpana Chawla and six of her colleagues, including Israel's first astronaut Ilan Ramon, were killed when the shuttle broke into pieces in the air over Texas on Saturday night, just 16 minutes before its scheduled landing at the Kennedy Space Centre in Cape Canaveral

US space agency NASA has begun internal and external independent investigations into the mishap.

The shuttle debris rained down over hundreds of square miles of the states of Texas, Louisiana and Arkansas, smashing a rooftop and splashing into a reservoir.

Human remains believed to be from the crew of the shuttle were found in Hemphill, Texas.

"I can confirm human remains from the space shuttle Columbia have been found in the debris," Karen Steele, spokeswoman for the Hemphill police, said on Saturday.

A piece of tile fell within 75 miles of President George W Bush's ranch in Crawford.

Debris has been tracked in a 500-square mile area but could be scattered over a region three times that, James
Kroll, director of the Emergency Geospacial Mapping Centre at the Stephen F Austin State University, said.

There were no reports of any injuries from the falling debris. The army has sent helicopters and soldiers to locate
the debris and guard the wreckage of the shuttle.

US authorities urged the people to report finding any debris but do not touch it for fear of contamination from
toxic substances.

Within minutes of Columbia's loss, computer records were impounded together with documentation relating to the pre-flight preparation for the shuttle. The information could be crucial in discovering what went wrong with the shuttle.

NASA lost contact with the shuttle as it hurried over Texas. The space agency has suspended shuttle flights until answers are found about how and why the disaster occurred.

"My promise to the crew and the crew families ... is we'll find the cause. We'll fix it, and we'll move on," Bill Readdy, NASA associate administrator for spaceflight, told a press conference.

Israeli ambassador to the United States Daniel Ayalon arrived in Houston on Saturday with the mourning family of Colonel Ramon, saying it was 'a very tragic day for Israel and America."

He said he and Ramon's family were appreciative of the 'outpouring of condolences' expressed by US officials. "We
thank NASA very much for the sensitivity and compassion that they showed with the family and with us."

He also expressed his country's condolences to the American people and the families of the six other astronauts killed.

In a briefing, NASA said the first indication of trouble was the loss of a temperature sensor readings in the left wing about seven minutes before all contact with the shuttle was lost. The tiles reflect the heat encountered during reentry into the Earth's atmosphere and experts say damage to them could result in exceptionally high temperatures that could damage the shuttle's structure. 

Till the failure of sensors was detected on the left wing, the mission was picture perfect and the shuttle was expected to land in beautiful bright weather.

Damage to tiles was one possibility and would be examined, NASA administrator Sean O'Keefe said.

"But there could be other reasons too. Sometimes, the obvious might be the real reason. They (investigators) would go through miles and miles of data and examine debris to pin point the cause."

In Cape Canaveral, shuttle director Ron Dittemore said: "as we look at that (the lift-off incident) now in hindsight, we can't discount that there might be a connection."

"We are mobilizing our forces, our engineers, our technicians, our best experts, to understand what went wrong," Dittemore said.

Chief flight director Milt Heflin said one of eight sensors sent a reading that the astronauts saw on their cockpit displays.

The astronauts mentioned a problem with tyre pressure, apparently following their usual procedure in acknowledging a reading of some concern, Heflin said.

Then a controller in Houston said, "We did not copy your last (message)."

"Roger, but ..." came the reply, followed by a crackling sound and then silence, as if the astronaut had been cut off in mid-sentence.

Columbia was the first shuttle to fly into space in 1981 and it was returning to earth after its 28th voyage when the
disaster occurred.

It was still unclear the kind of setback the Columbia tragedy would cause to the space programme. As of now, NASA has suspended all shuttle flights pending an investigation. 

The International Space Station is not dependent on US shuttle missions and can use Russian Soyuz spacecraft.