The chairman of the commission probing the space shuttle Columbia disaster told lawmakers yesterday that investigators are closing in on the precise cause of the accident and may issue preliminary findings as soon as the next month or two, several participants in the meetings said.

"We think they're close to causation," said Rep. Ralph M. Hall (Tex.), the ranking Democrat on the House Science Committee, following a meeting on Capitol Hill with retired Adm. Harold W. Gehman Jr., chairman of the investigative panel.

Gehman declined to speculate on the exact cause of the breach of the shuttle's left wing that triggered the accident, which killed the seven crew members, but indicated that investigators "have narrowed it to the neighborhood," added Rep. Bart Gordon (D-Tenn.).

Republican and Democratic lawmakers were also shown a new videotape of the most comprehensive schematics and photographs assembled to date plotting the shuttle's fatal reentry into Earth's atmosphere, showing how it began shedding pieces and finally disintegrated over Texas early Feb. 1, only minutes from landing in Florida.

The videotape -- which has not been made public -- incorporates a matrix of data from shuttle sensors and photographs taken from the ground by government agencies and private individuals. The tape documents that the shuttle began to break up much earlier than NASA officials had originally assumed.

Last Friday, NASA announced that a tile from the shuttle was found in Littlefield, in the Texas Panhandle, more than 300 miles farther back along the flight path than where other pieces have been found. Investigators said they have credible imagery showing that the shuttle may have begun losing parts even as it approached the California coast, although the evidence is not conclusive.

Yesterday, a number of lawmakers emerged from meetings with Gehman saying they were startled by how soon the shuttle had begun to break up and amazed that it had survived some of the hottest temperatures of the mission, only to disintegrate shortly before landing.

"It was clear they were 75 percent home free when the thing really started going haywire," said Rep. Dana Rohrabacher (R-Calif.), chairman of the House space and aeronautics subcommittee. "Just a few more minutes or a few more seconds even and they might have been past the danger point. It was very sad."

NASA has also recovered a heat-damaged fragment of a videotape, most likely shot by Columbia astronaut Laurel B. Clark just minutes before the shuttle disintegrated, that shows an untroubled crew headed into Earth's atmosphere. The astronauts' families have seen the tape, but it was not shown to members of Congress during yesterday's sessions.

Gehman spent much of the day briefing key members of Congress and leadership aides to update lawmakers on the probe and to bolster confidence in the independence of his commission.

The commission was appointed by NASA Administrator Sean O'Keefe within hours of the shuttle accident, and until recently it depended heavily on NASA officials for data analysis and support. Some lawmakers complained that the board was too reliant on NASA and that space agency officials were in a position to influence the direction of the probe.

O'Keefe subsequently rewrote the commission's charter three times to bolster its independence and promised to expand the nine-member panel with three experts from outside the government.

Sheila Widnall, a Massachusetts Institute of Technology aerospace scientist and secretary of the Air Force during the Clinton administration, was added last week, and Gehman told lawmakers yesterday that two more will be appointed within the next week or two.

House Science Committee Chairman Sherwood L. Boehlert (R-N.Y.) said Gehman "was very persuasive" that the board is operating independently and that he is fast replacing NASA support staff with outside experts.

"The perception in some sectors is that the board as initially structured was not independent," Boehlert said. "The reality is that I'm convinced that it is independent."

Hall, the ranking Democrat, said he was impressed with the progress of the investigation and agreed with Boehlert that Gehman is proceeding independently of the space agency.

However, Gordon, the ranking Democrat on the space and aeronautics subcommittee, said he remains concerned that NASA engineers and officials are still filtering critical data. He and a few other members of Congress have urged President Bush to appoint a blue ribbon commission -- similar to the one that investigated the 1986 Challenger shuttle disaster -- to supercede the Columbia investigation.

"I think [Gehman] is trying to throw off some of the shackles that he inherited with that flawed system," Gordon said. "But I think you still have that flawed system with a number of filters that is going to make it uncomfortable for some whistle-blowers and others to come forward."