Senior members of the congressional panels investigating the Iran-contra affair have concluded that Richard V. Secord, Elliott Abrams and Fawn Hall did not tell the whole truth when they were questioned at recent public hearings, congressional sources said yesterday.

The sources also disclosed that a fourth witness, former national security adviser Robert C. McFarlane, was called back by the committees this week for private questioning to clarify testimony conflicting with that of others concerning an unspecified 1985 document. McFarlane was not under oath during his second appearance. And the committees now appear satisfied that the conflict has been resolved.

Regarding the testimony of others, however, a committee member said, "I think we're being lied to under oath by some people."

Secord, the chief private collaborator of fired White House aide Oliver L. North on the Iran and contra operations, was recently called back for further interrogation in closed session to answer questions that have arisen since he was the leadoff witness at the hearings in early May, sources said.

Abrams, the assistant secretary of state for inter-American affairs, came under sharp attack from committee members after he repeatedly insisted that he was unaware of Marine Lt. Col. North's activities in support of the contras in 1985 and 1986, despite frequent meetings with North. Hall, North's secretary at the National Security Council, testified that North never told her why he directed her to alter or destroy key NSC documents last Nov. 21.

"I always look back to the dance of the seven veils," one committee member said. "Most people take off three veils. Before the investigating committee, at most they took off four veils -- and I would say almost without exception, if you didn't ask the right question you did not get a response."

The first phase of the hearings, in which 18 witnesses testified for nearly six weeks, ended last week. In looking ahead to the second half of the Iran-contra hearings, congressional sources say they expect the testimony to be damaging not only to President Reagan but also to Cabinet members and their departments.

The sources describe members as buoyed by growing public support for the inquiry, and in no mood to accept the view that some testimony should be avoided "because it might hurt the country," as one source put it.

"Right now the CIA is being ripped apart," the source said. He said that CIA Director William H. Webster would be called "not just because he was involved in this conspiracy, or was aware of what was happening, but to give him an opportunity to rehabilitate his agency in living color."

There has been extensive testimony of CIA support for the secret contra war against the government of Nicaragaua at the top and the bottom of the agency. North, who directed the private effort on behalf of the contras, kept the late CIA director William J. Casey informed, and the CIA station chief in Costa Rica has testified that he communicated directly with North. At the time, Congress had banned military aid to the contras by U.S. intelligence agencies.

Committee investigators yesterday concluded 20 hours of private questioning of John M. Poindexter, the president's national security adviser through most of 1986.

The congressional sources expect his public testimony next month to be "very, very explosive," and to provide a "major story from the first day." Committee spokesmen declined to elaborate.

Poindexter's testimony about the crucial stages of the Iran and contra initiatives will lay the groundwork for public questioning of Secretary of State George P. Shultz, Defense Secretary Caspar W. Weinberger, former White House chief of staff Donald T. Regan, Attorney General Edwin Meese III and Deputy CIA Director Robert Gates.

Investigators say that with Casey's death last month, they lost "a great deal" in terms of their ability to learn what he told the president.

But evidence already has convinced some key members of the committees that North worked for Casey as much as for his official boss, the national security adviser.

One source said testimony by McFarlane suggests that Casey used North without first consulting McFarlane to shift a controversial hostage ransom plan from the CIA to the Drug Enforcement Administration in 1985.

When Sen. Warren B. Rudman (R-N.H.) questioned McFarlane about the plan, McFarlane reacted with what many spectators thought was inexplicable anger.

Sources now believe that the questions touched a sensitive nerve about North's special relationship with Casey. At the time, McFarlane may not have been aware that there was a pre-existing ransom operation at the CIA known to North but not to him. Only when North needed McFarlane to seek approval from the president and attorney general for shifting the operation to DEA was McFarlane told.

The transfer, the source noted, "was not for operational reasons because there are . . . DEA agents in Cyprus," but because the administration officials believed there was no DEA accountability to the intelligence committees.

The committees yesterday announced that public hearings next week will center on U.S. arms sales to Iran and will call as witnesses former NSC consultant Michael Ledeen, former CIA counsel Stanley J. Sporkin, Assistant Attorney General Charles Cooper, former deputy secretary of defense Noel C. Koch, Colin Powell of the NSC, Henry Gaffney of the Defense Security Assistance Agency and former CIA employe Glenn Robinette.