JOHNSON SPACE CENTER, Texas (CNN) -- Family members and friends mourned the crew of the space shuttle Columbia on Sunday as NASA's chief vowed the agency would "make sure this never happens again."
Thousands of fragments of the shuttle awaited discovery in a 200-mile swath of eastern Texas and western Louisiana a day after the craft disintegrated more than 1,000 miles short of its destination. All seven astronauts aboard were killed
"We owe it to them every single second of the day to be sure we dedicate ourselves to finding out what went wrong," NASA Administrator Sean O'Keefe said Sunday. "We're going to find out what caused this, we're going to make sure we correct it, and we're going to make sure this never happens again."
The shuttle crew was remembered during Sunday church services. In Amarillo, Texas, at shuttle commander Rick D. Husband's church, family friend Patti Ragan remembered Husband as "a man of God" who "put himself into everything he did with a full heart." (More on memorials)
In Racine, Wisconsin, friends and family members mourned mission specialist Laurel Clark at Olympia Brown Unitarian Universalist Church.
"Laurel was a very intense person who would set goals and would go for them," Clark's brother, Daniel Salton, said Sunday. "And I think that's a great role model for kids today. ... You can do great things for humanity if you just set some small goals and always go for the next thing and set your sights higher."
Those killed on the ill-fated shuttle flight were Husband, Clark, pilot William C. McCool, payload commander Michael P. Anderson, mission specialists David M. Brown and Kalpana Chawla, and Israel's first astronaut, Ilan Ramon.
"Columbia's lost. There are no survivors," a grim-faced President Bush said in an address to the American people Saturday. "These astronauts knew the dangers and they faced them willingly, knowing they had a high and noble purpose in life." (Full story)
Human remains were found in the eastern Texas county of Sabine and in Hemphill, the county seat. An intact mission patch bearing the names of the seven crew members was also discovered. (Full story)
Even as they mourn for their seven friends and colleagues killed in the shuttle disaster, NASA officials have begun investigating why Columbia broke into pieces.
"My promise to the crew and to the crew families is that the investigation that we have just launched will find the cause, we'll fix it and then we'll move on. We can't let their sacrifice be in vain," said NASA Associate Administrator Bill Readdy, a veteran of two shuttle flights. (Transcript)
There were indications of a loss of tire pressure on the left main gear, then indications of excessive structural heating before the shuttle disintegrated, said shuttle program director Ron Dittemore. (More on investigation)
Officials said they will take another look at a piece of foam that came off during takeoff. (Disaster timeline)
A NASA official said the shuttle's altitude made it "highly unlikely" that it was a victim of a terrorist act. FBI officials quickly discounted the possibility of foul play or terrorism. (Full story)
Space shuttle flights have been put on hold until NASA can learn what caused the disaster. NASA said the international space station, where two astronauts and a cosmonaut remain, has enough supplies to last the crew until June.
The next shuttle flight had been scheduled for March 1.
Parts of the shuttle -- large and small -- were found alongside highways, in parking lots, on sidewalks and front lawns. There were 800 debris sites in Nacogdoches County, Texas, alone.
In Hemphill, about 75 miles away, emergency operations official Bill Ted Smith said the search for debris was "painstakingly slow."
"We have approximately a 250 square-mile-path that we're having to search. We got around 40 square miles yesterday identified, and due to the rugged forest terrain that is around here, it's making the search difficult," he said.
People were urged not to go near the debris because it could contain toxic substances from the shuttle fuel.
No injuries were reported from the falling debris, which Nacogdoches Mayor Roy Blake called "a miracle."
Texas Gov. Rick Perry said Columbia left a debris field stretching from the Dallas area northward to the Arkansas border and eastward to Alexandria, Louisiana. Perry said most of the debris seemed to be about 2- to 5-inch pieces.
Witnesses in Texas reported debris falling from the sky accompanied by a cascade of thunder. Heat-detecting weather radar showed a bright red streak moving across the wide Texas sky.
"When it got nearer, we could see flecks or pieces coming off of it," said Linda Steed, who lives near Nacogdoches. She said the sound "reverberated" for several minutes, "like a rolling thunder."
"I'm devastated. It's unbelievable. It makes me so sad," she said.
Officials asked anyone who finds debris to call (281) 483-3388.
Columbia was lost less than a week after the anniversaries of two other deadly space program disasters -- the 17th anniversary on January 28 of the explosion of the shuttle Challenger in 1986 and the 36th anniversary on January 27 of a launch pad fire that killed three Apollo astronauts in 1967.