Addressing the nation Saturday afternoon, President Bush said, "the Columbia is lost. There are no survivors."
"These men and women assumed great risk in the service to all humanity," Mr. Bush said. "It is easy to overlook the dangers of travel by rocket... These astronauts knew the dangers, and they faced them willingly knowing they had a high and noble purpose in life."
"The cause in which they died will continue. Mankind is led into the darkness beyond our world by the inspiration of discovery and the longing to understand," the president added. "Our journey into space will go on."
Speaking to the crew's families, the president said, "Our entire nation grieves with you. And those you loved will always have the respect and gratitude of this country."
NASA Administrator Sean O'Keefe said a rapid response team had been dispatched to northeast Texas, where debris from the flight has been discovered. O'Keefe also said that information from the mission had been secured and an external board made up of Air Force, Navy and Department of Transportation personnel, among others, will investigate the incident.
O'Keefe said he alerted President Bush shortly after NASA officials lost contact with the Columbia just after 9am Eastern time. He said the president had since spoken with the shuttle crew's family members and offered NASA the government's full and immediate support.
The Federal Emergency Management Agency will head up effort to recover shuttle debris, O'Keefe said.
NASA officials did not discuss what might have caused the shuttle to break apart, however O'Keefe said there was "no indication the mishap was caused by anything or anyone on the ground."
Earlier Saturday, Homeland Security Department spokesman Gordon Johndroe told reporters there was no indication of foul play or terrorism involved in the shuttle disaster.
"Obviously the investigation is just beginning, but that is the information we have now," Johndroe said.
Later Saturday, Chief Flight Director Milt Heflin said one sign of trouble was the loss of data from temperature sensors in the left wing's hydraulic system. Shuttle program manager Ron Dittemore said shuttle fights are on hold while an investigation into the incident continues.
"We cannot yet say what caused the loss of Columbia," Dittemore told reporters. "It will take us some time to work through."
Some Texas residents said they saw flames in the sky and heard a "big bang" over the eastern area of the state at about 9am, the same time that officials lost contact with the craft. It had been scheduled to land in Florida at 9:16am.
Press reports quoted residents in Nacogdoches, Texas, a city some 135 miles north of Houston, as saying parts of machinery and other debris has been found there.
NASA officials warned residents to stay away from any shuttle remnants because of the "toxic propellants used aboard the Space Shuttle" and to report any debris to authorities.
The crew included six Americans and Israel's first astronaut, whose mission had stirred excitement in his home country.
"The state of Israel and its citizens are as one at this difficult time," Prime Minister Ariel Sharon's office said in a statement Saturday.
O'Keefe praised the crew as "an extraordinary group of people," and said "the loss of this valiant crew is something we will never be able to get over."
The crew had completed more than 80 scientific research experiments while in orbit. The disaster is the first accident in 42 years of manned space flight during the descent to Earth or landing.
The last major disaster in the space program was the 1986 explosion of the Space Shuttle Challenger. The Challenger exploded 72 seconds after takeoff from Cape Canaveral. All seven crew members, including a schoolteacher, died in the incident.
Columbia was NASA's oldest shuttle and this had been its 28th flight. The mission was the 113th flight in the shuttle program's history.