Senior members of the Senate Intelligence Committee say they were shocked to find that a huge new spy satellite headquarters under construction outside Washington would cost up to $350 million. They said the Pentagon and CIA had concealed the full expense of the project from them.

"You've got to see it to believe it," said Sen. John Warner, of Virginia, the ranking Republican on the committee. "I was absolutely astonished at the magnitude and the proportions of this structure."The project in question is a 1 million-square-foot complex near Chantilly, Va., close to Dulles International Airport, being built to house about 3,000 contractors and government workers employed by the National Reconnaissance Office, the nation's most secret intelligence agency. Its cost, by comparison, exceeds the `rebuilding' of New York City's Pennsylvania Station; its size is about one-fifth of the Pentagon's.

"Has this process created a Taj Mahal?" Warner asked rhetorically. "We don't know."

The existence of the National Reconnaissance Office was a state secret until late 1992, and almost nothing is known about the office, other than its mission of building the nation's spy satellites. Its annual budget, secretly appropriated, buried within the Pentagon's accounts in the so-called "black budget" and never publicly disclosed, has been estimated at $6 billion, or about three times the budget of the entire State Department.

It appears that the new complex was buried so deeply and concealed so successfully inside the NRO's secret appropriations that the ranking members of the Senate Intelligence Committee were, in their words, "shocked and dismayed to learn" its real cost. They now say that the true sum was "never effectively disclosed to our committee."

Warner said that "someone, or some group of persons, conceived of a means by which to take a project and build it" by taking "a very stealthy course."

Martin C. Faga, director of the National Reconnaissance Office from September 1989 to March 1993, said: "It was a stealthy course, of course - purposefully so. But that was a reason why it was discussed in detail with the Intelligence Committee." Almost all of the briefings given to the Senate and House Intelligence Committees take place in secret.

"I don't think there's any doubt the committee knew the facility was being built," Faga said in an interview. "We briefed them in '90, '91, '92. But that doesn't mean the committee understood what it was going to cost. These are complex projects. It's perfectly plausible that folks were looking at pieces of the budget, not looking at other pieces, not seeing that there's an aggregate cost there. I can easily imagine that they did not recognize what was going on."

Sen. Dennis DeConcini, the Arizona Democrat who heads the Intelligence Committee, said: "There was definitely an effort not to disclose the cost and the expense - even the location - for a long period of time." For that, he blamed "the culture and the philosophy in much of the intelligence community that `we don't have to account' like anybody else does."

The handful of private analysts who try to track the National Reconnaissance Office and its satellites expressed astonishment at the committee's outcry. "There's a third of a billion-dollar puzzle palace out there in Virginia and nobody noticed?" said John Pike, director of space policy at the Federation of American Scientists. "What we have here is a failure to communicate."

On Monday, in a written statement, the director of central intelligence, R. James Woolsey, and the Pentagon, said that their investigators will "review the history of this project."

The Central Intelligence Agency and the Pentagon share responsibility for running the National Reconnaissance Office.