Hundreds of police and soldiers are searching a huge area of Texas and Louisiana for wreckage of the space shuttle Columbia.

Much of the debris came down in a forested area of Texas called Piney Woods, forcing searchers to use horses and off-road vehicles to navigate the difficult terrain.

	:	Shuttle disaster
How Columbia broke up over Texas

More than 500 fragments of Columbia - ranging in size from a large car to a pea - have already been recovered.

So have some human remains, although early reports that the bodies of all seven astronauts had been recovered seem to have been premature.

The US space agency (Nasa) is warning people not to touch any debris they find.

They are concerned about the risk of injury or of damage to potentially useful evidence.

Several dozen people have already required hospital treatment.

Officials from 30 agencies - including the FBI, Federal Emergency Management Agency and National Transportation Safety Board - will fan out to help in the search.

The Environmental Protection Agency and federal hazardous materials crews will also be on hand.

'Crime scene'

The BBC's David Willis in Texas said the search was like that of a crime scene, with Nasa scientists acting as detectives.

Officials document everything they find

Nasa will rely on experts in forensics, engineering and airline accidents.

The area the searchers need to cover is very large. Texas Governor Rick Perry said wreckage had been found in 33 counties across the state.

Debris was scattered over an area 160 km (100 miles) long and 16 km (10 miles) wide.

A team of more than 250 searchers covered one half of 1% of the total area on Sunday, the Daily Sentinel newspaper of Nacogdoches, Texas, reported.

SHUTTLE CREW

Commander Rick Husband, US Pilot William McCool, US Michael Anderson, US David Brown, US Kalpana Chawla, US Laurel Clark, US Ilan Ramon, Israel

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Officials do not have enough personnel to guard all the pieces of wreckage until they can be collected.

They are to be taken by truck to Barksdale Air Force Base in Louisiana, where Nasa has established a command post.

Engineers from United Space Alliance, the shuttle contractor, will then study the debris for clues about what caused Saturday's disaster.