Newly disclosed e-mails inside NASA showed senior engineers worried a day before the Columbia disaster that the shuttle's left wing might burn off and cause the deaths of the crew, a scenario remarkably similar to the one investigators believe actually occurred.

The dozens of pages of e-mails describe a broader, internal debate than previously acknowledged about the seriousness of potential damage to Columbia from a liftoff collision with foam debris from its central fuel tank. Engineers never sent their warnings to the National Aeronautics and Space Administration's brass.

Engineers in Texas and Virginia fretted about the shuttle's safety during its final three days in orbit. One speculated whether officials were "just relegated to crossing their fingers" and another questioned why such dire issues had been raised so late.

"Why are we talking about this on the day before landing and not the day after launch?" wrote William C. Anderson, an employee for the United Space Alliance LLC, a NASA contractor, less than 24 hours before the shuttle broke apart Feb. 1 while returning to Earth.

NASA said those messages -- including the few that were hauntingly prescient -- were part of a "what-if" exercise by engineers convinced the shuttle would land safely despite possible damage from foam that struck insulating tiles on the spacecraft's left wing at liftoff.

"It was a surprise to us when the 'what-if' scenario played out," said Robert Doremus, head of the mechanical systems group in Mission Control. "We were not expecting that."

In Washington, NASA Administrator Sean O'Keefe told the House Science Committee on Thursday that, with shuttle launches on hold indefinitely, negotiations are under way with Russia to use Soyuz spacecraft to bring home and replace the crew aboard the international space station.

O'Keefe also said the independent board investigating the disaster "has made significant progress in organizing its work to determine the cause of the accident."

The engineers' e-mails also showed that the space agency was sufficiently concerned about possible damage to Columbia that it asked the Defense Department -- then abruptly changed its mind -- to take pictures of the shuttle in orbit more than one week before its breakup.

The request came six days into the mission, on Jan. 22, for the U.S. Strategic Command to take satellite images of suspected damage to the shuttle's left wing. For weeks until Wednesday, NASA has denied it ever made such a request.

The space agency withdrew its informal request one day later amid fears it might have "cried wolf" and endangered future such requests, according to one e-mail.

Deciding against the satellite request, a space official wrote reassuringly to the Defense Department that Columbia was "in excellent shape" and that insulating foam that struck the shuttle on its mid-January liftoff was "not considered to be a major problem."

Not everyone agreed.

Three days before the end of the doomed mission, one frustrated engineer, Robert Daugherty, asked, "Any more activity today on the tile damage or are people just relegated to crossing their fingers and hoping for the best?" The response: "I have not heard anything new."

After intense debate -- occurring by phone and e-mails -- the engineers, some supervisors and the head of the space agency's Langley research facility in Hampton, Va., decided against taking the matter to top NASA managers, including William F. Readdy, NASA's associate administrator for space flight.

Jeffrey V. Kling, a flight controller at Johnson Space Center's mission control, foresaw what might happen to Columbia during its fiery descent: that superheated air could penetrate the wheel compartment and cause the wing to fail.

Kling wrote just 23 hours before the disaster that his engineering team's recommendation in such an event "is going to be to set up for a bailout (assuming the wing doesn't burn off before we can get the crew out)." The following day, Kling was among the first in mission control to report a sudden, unexplained loss of data from the shuttle's sensors in the left wing.

"This was just a mental exercise that we went through to 'what-if' the whole thing," Kling said.

The shuttle crew had individual parachutes, but the chutes wouldn't have done them any good at the speed and altitude they were flying when Columbia broke up.

The e-mails showed the debate was triggered by a telephone call Jan. 27 to Daugherty from Carlisle Campbell, a NASA engineer at Johnson Space Center, about how re-entry heat could damage the shuttle's tires.

Another e-mail, from R.K. "Kevin" McCluney, a shuttle mechanical engineer at the Johnson center, described the risks that could lead to "LOCV" -- NASA shorthand for the loss of the crew and vehicle.

McCluney ultimately recommended to do nothing unless there was a "wholesale loss of data" from sensors in the left wing. Investigators reported such a wholesale loss of sensor readings in Columbia's left wing, but it occurred moments before the shuttle's breakup -- too late to do anything.