CAPE CANAVERAL, Fla. -- Seven astronauts and NASA's first space shuttle were presumed lost over the skies of Texas on Saturday as Columbia was re-entering Earth's atmosphere to wrap up a 16-day science mission.

A Texas television station captured video of the shuttle breaking up into multiple pieces as the vehicle flew at an altitude some 200,000 feet and was moving about 12,500 mph.

Search and rescue forces in the Dallas and Ft. Worth area were put on alert and NASA officials warned citizens to avoid any debris found on the ground. Such debris can include traces of toxic chemicals and be extremely hazardous.

Presumed lost were commander Rick Husband, pilot Willie McCool and mission specialists Kalpana Chawla, Laurel Clark, Mike Anderson, David Brown and Israeli payload specialist Ilan Ramon.

The flight crew were wearing partial pressure suits, each equipped with a parachute, but the chances of surviving a break up at that height and under those extreme conditions are remote.

At Mission Control in Houston, flight controllers were in full contingency mode, securing data, making copies of files and locking up log books and any other form of information that might be helpful to the investigation into what went wrong.

Here's what we know so far about what happened:

Routine pre-landing operations began in earnest this morning at 5:35 a.m. EST (1035 GMT) when Husband and McCool closed the orbiter's payload bay doors and then loaded the entry and landing software into the shuttle's main computers.

The entire crew then continued through their checklists as scheduled, donning their orange spacesuits, strapping themselves into their seats and drinking fluids that would help their bodies re-adapt to gravity after touchdown.

Weather over the Florida landing site was nearly ideal, with ground fog the only concern. As the sun came up, the fog disipated and Mission Control gave Columbia a go for the de-orbit burn.

The twin orbital maneuvering system engines fired right on time at 8:15 a.m. EST (1315 GMT), committing the shuttle to its hypersonic dive back through Earth's atmosphere.

At about 8:55 a.m. EST (1355 GMT) the shuttle crossed the California coast high above San Francisco, continuing on above Nevada, Arizona, New Mexico and Texas.

No problems were reported at any point up to that point.

At 9 a.m. EST (1400 GMT) Mission Control lost contact with the shuttle.

About 9:05 a.m. EST (1405 GMT) Mission Control began trying to communicate with Columbia's crew through the UHF radio at the Merritt Island Tracking Statio here at Cape Canaveral, but to no avail. Neither voice nor telemetry was received.

There was no indication as to what might have happened.

At the pre-landing news conference, entry flight director answered questions about potential damage to Columbia's heat protection tiles that might have been the result of insulating foam seen coming loose from the shuttle's external tank at launch.

Cain said engineers studied the situation and were not worried.