Excerpts from remarks made by NASA Administrator Sean O'Keefe and Bill Readdy, NASA's associate administrator for space flight, at the agency's first news briefing on the Columbia disaster:

O'Keefe: This is, indeed, a tragic day for the NASA family, for the families of the astronauts who flew on STS-107 and, likewise, tragic for the nation.

Immediately upon indication of a loss of communications on STS-107, at a little after 9:00 a.m. this morning, we began our contingency plan to preserve all the information relative to the flight activities.

I immediately advised the president and the secretary of homeland security, Secretary Tom Ridge, at the point after landing was due to have occurred at 9:16, spoke to them very briefly thereafter to advise that we had lost contact with the shuttle orbiter Columbia and STS-107 crew.

They offered -- the president specifically offered the full and immediate support to determine what the appropriate steps were thereafter to be taken.

We then spent the next hour and a half working through the detail and information of what we have received, and Bill Readdy will walk you through the specifics of those operational and technical issues here in just a moment.

Thereafter, we have met with the family members of the astronauts who were here at Kennedy Space Center and are soon to be departing back to Johnson, to Houston.

The president has called and spoken to them to express our deepest national regrets. We have assured them that we will begin the process immediately to recover their loved ones and understand the cause of this tragedy.

At this time, we have no indication that the mishap was caused by anything or anyone on the ground.

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O'Keefe: Here this morning with the families of the astronauts and their friends started out as a pretty happy morning, awaiting the landing of STS-107. And we had highly anticipated their return because we couldn't wait to congratulate them for their extraordinary performance and the excellent efforts on the science mission on this very important flight.

They dedicated their lives to pushing the scientific challenges for all of us here on Earth, and they dedicated themselves to that objective and did it with a happy heart, willingly and with great enthusiasm.

The loss of this valiant crew is something we will never be able to get over. And certainly the families of all of them we have assured we will do everything, everything we can possibly do to guarantee that they work their way through this horrific tragedy.

We ask the members of the media to honor that too, to please respect their privacy and please understand the tragedy that they are going through at this time. We will help the media assure that be the case as well.

We trust that the prayers of the nation will be with them and with their families. And again, a more courageous group of people you could not have hoped to know than the families of these crew members. An extraordinary, extraordinary group of astronauts who gave their lives and did it in a way that they knew exactly the risk but never in a--ever do we ever want to see a circumstance where something like this could ever happen.

And we diligently dedicate ourselves every single day to assuring these things don't occur. And when they do, we have to act responsibly, accountably, and that's exactly what we will do.

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Readdy: This is a truly difficult day for all of us. Many of us were standing alongside the runway waiting to celebrate their triumphant return after a 16-day science mission.

I think you could tell from the down link that they loved what they were doing and they thought what they were doing was extremely important, pushing back those boundaries in science.

At 9 o'clock we heard that they had lost data from the space craft, and it appears that that was at about 200,000 feet and about mach 18.

The loss of data was somewhere over north-central Texas. And at the planned landing time of 9:16 we initiated our contingency action plan, called the Rescue Coordination Center and initiated a search and rescue effort.

Sadly, I think from the video that's available, does not appear that there were any survivors.

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Readdy: The immediate focus is on the crew families, and we spent some time with them. The president called. I'd have to say the families are bearing up with an incredible amount of dignity, considering their loss.

We all grieve for them, we all pray with them for the crew. But one thing came across loud and clear when visiting with them, is they knew that the crew was absolutely dedicated to the mission that they were performing. And I think you could see that in the video down link. They believed in what they were doing.

And in the conversations with the crew and their families, they said that we must find what happened and fix it and move on, and we can't let their sacrifice be in vain.

Today was a very stark reminder that this is a very risky endeavor, pushing back the frontiers in outer space. And after 113 flights, unfortunately people have a tendency to look at it as something that is more or less routine. Well, I can assure you, it is not. Each and every time I flew, each and every time my colleagues flew, we treated that with the respect it deserved from a professional standpoint.

And I have to say that, as the one responsible for shuttle and station within the NASA, that I know that the people of NASA did everything possible preparing for this flight to make it as perfect as possible.

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.

Thank you.