Pieces of space shuttle Columbia were found in several east Texas counties and in Louisiana. Debris has been tracked in a 500-square-mile area but could be spread over a region three times that.

People finding debris are asked to call the following numbers to report the discovery: Gov. Rick Perry's hotline -- 800-525-5555 or NASA -- 281-483-3388.

• Map: Witness/Debris Sightings

NASA has asked members of the public to help in its search for debris, but has warned people not to touch the pieces because they might be contaminated with toxic propellants.

Shuttles have long used a chemical called hydrazine to run their auxiliary power units. Hydrazine, a colorless liquid with an ammonia-like odor, is a toxic chemical and can cause harm to anyone who contacts it.

The Associated Press contributed to this report.