CAPE CANAVERAL, Fla. _ NASA told a nation in mourning Sunday that remains of all seven astronauts from shuttle Columbia had been recovered, and that investigators were beginning to zero in on the cause of the tragedy.

“We found remains from all the astronauts,” said Bob Cabana, NASA’s director of flight crew operations. “It’s still in the process of identification.”

Other NASA officials said that temperature readings on the left side of Columbia rose precipitously six minutes before it disintegrated, and the spaceship struggled for two minutes to maintain a safe flight path during the hazardous descent to Earth.

As the space agency appointed two investigative teams and search crews combed more than 1,400 debris fields in Texas and Louisiana, Ron Dittemore, the shuttle’s program manager, sketched a fuller picture of what he called “a significant thermal event” aboard Columbia. He said it boosted temperatures on the exterior of the shuttle by 60 degrees within five minutes, an unprecedented event.

His account suggested that the evidence so far points to a breach in the heat protection system along Columbia’s left side, particularly on the left wing, which was hit during liftoff by insulating foam from an external fuel tank.

Speculation is now focusing on the possibility that crucial thermal insulating tiles along that wing were damaged or lost during liftoff, flight or re-entry.

“We’re piecing together the puzzle and we are beginning to make progress,” Dittemore told a news conference at the Johnson Space Center in Houston.

Still, he cautioned against a rush to judgment, saying that engineers were looking at many possible causes of the accident and that it was far too early to draw any conclusions.

“I don’t have a smoking gun,” he said. “I don’t have a root cause … I haven’t ruled out anything.”

Columbia disintegrated 39 miles over Texas as it streaked toward a landing Saturday morning at Cape Canaveral, Fla.

Falling pieces of the spaceship showered fields, roads, lakes and backyards in an area 100 miles long and 10 miles wide, complicating a recovery effort of exceptional proportions.

Killed were shuttle commander Rick Husband and crewmates Michael Anderson, David Brown, Kalpana Chawla, Laurel Clark, William McCool and Ilan Ramon, Israel’s first astronaut.

Dittemore said he believed the flight crew realized that something was amiss, but that the astronauts could not have known how serious it was.

“We have no data, no communications, no evidence that the crew was alarmed,” he said.

NASA also disclosed that it had attached a phased-out, relatively heavy version of the fuel tank to the shuttle for this flight. No evidence has surfaced suggesting a link between that and the accident.

Dittemore said sensors on the left side of the shuttle showed a temperature increase of nearly 30 degrees within five minutes as the craft passed over California at 8:53 a.m. EST, six minutes before it disintegrated.

One minute later, he said, sensors showed that the temperature had risen 60 degrees during the preceding five minutes.

At 8:58 a.m., he said, with Columbia now over New Mexico, instruments showed an unusual amount of aerodynamic drag along the left side of the shuttle. At 8:59 a.m., one minute before the shuttle disintegrated, that drag increased significantly.

Dittemore said the shuttle’s automated flight systems worked successfully to put keep the craft on course, but nothing like this had ever been seen during the 22-year shuttle program.

“It’s out of family,” he said. “We’ve never seen it to this degree, but it does not approach the limits of the flight control system.”

He said NASA was “intrigued” by a report submitted by an observer in California who said he saw something separate from the shuttle as it passed overhead at about the time the sensors first reported unusual temperature readings.

As NASA began collecting information, a grieving nation struggled to absorb the loss of Columbia and its seven astronauts.

“We grieve because they represented the best in us, because part of us has died,” the Rev. Luis Leon told President Bush, his wife, Laura, and other congregants at St. John’s Episcopal Church, a block from the White House. “I believe God’s heart is more broken than our own.”

The White House announced that Bush would attend a memorial service Tuesday at the Johnson Space Center near Houston.

Rep. Bill Nelson, D-Fla., who was briefed by NASA late Saturday, said the discovery of body parts on the ground suggested that the crew compartment probably remained relatively intact during the accident, though it apparently was breached by debris and sustained rapid, catastrophic depressurization.

The shuttle’s crew compartment is designed to maintain some degree of structural integrity if catastrophe strikes.

“I hope and pray that their pressure suits would have been punctured at the same time to cause immediate loss of consciousness,” said Nelson, a member of the Senate subcommittee that deals with science, technology and space.

Dittemore said he had no information about the condition of the crew compartment.