The case has been in the courts for nearly a decade and Mr. Casolaro's brother, Dr. Anthony Casolaro of Arlington, Va., has told reporters he believed his brother may have been close to uncovering a major conspiracy in connection with the Inslaw case. He said in an interview today that his brother had told him in the last two months that if he died in an accident, "don't believe it."

Dr. Casolaro said he was very skeptical that his brother committed suicide for several reasons, including the facts that his brother had recently received numerous death threats and that none of his notes on the case were found with his body.

Friends of the journalist said he was looking into a connection between the Inslaw matter and the Bank of Credit and Commerce International, a loosely regulated international banking concern that Federal regulators say is at the center of a worldwide banking fraud. U.S. Inquiry Is Suggested

Elliot Richardson, a former United States Attorney General who now represents Inslaw in its suit against the Justice Department, said Mr. Casolaro's death should be the subject of a Federal investigation led by someone "of unquestioned integrity and independence."

Mr. Richardson said today, "The significant thing about Danny's death is that he was just seeking confirmation of what he believed he already knew." He said that if the informers Mr. Casolaro had already talked to were to be believed, it involved a conspiracy "far worse than Watergate," one that involved B.C.C.I., drugs and the persistent but unproven allegations that in 1980 some members of Ronald Reagan's Presidential campaign team worked to delay the release of American hostages in Iran to damage President Jimmy Carter's re-election chances.