The Justice Department announced yesterday that its nine-month special investigation of the Billy Carter case did not show illegal conduct on the part of any government official or employe.

In a 10-page final report, the department's chief internal investigator, Michael E. Shaheen Jr., said he still believes that Billy Carter lied to government agents who investigated his ties to Libya and that former attorney general Benjamin Civiletti had been deliberately "dissembling" last summer when he denied having talked to President Carter about the case.

Despite that, Shaheen found that Justice's inquiry, which ended with a court order requiring the president's brother to register as a foreign agent, had been thorough and competent.

The report also said there is no evidence that Civiletti "sought to obstruct or impede the [department] criminal division's handling of the Billy Carter investigation."

Shaheen said the criminal division now must decide whether to prosecute Billy Carter for lying to government agents and to resume monitoring his "continuing obligations" under the Foreign Agents Registration Act.

The division responded with a statement saying it would examine Shaheen's material and "reach a conclusion [as to] whether it warrants further action." Charges of lying to government agents are rarely prosecuted.

The final report, submitted to Solicitor General Wade H. McCree Jr., disclose that President Carter was interviewed under oath at the White House last Dec. 19 on lingering questions about his awareness of Billy Carter's activities.

"In response to questioning, President Carter stated that he had no advance warning, and little direct knowledge, of Billy Carter's dealings with the Libyans," the report stated.

The president acknowledged, "in retrospect," that his use of Billy to initiate contacts with Libyan diplomats in the fall of 1979 during the Iranian hostage crisis obviously would have enhanced Billy's prestige with the Libyans. But Jimmy Carter said he did not "focus" on that likelihood at the time.

"The president stated that he at no time discussed the release of [two Boeing 727] aircraft to Libya with his brother and that he did not receive any pressure from any family member regarding the release of such aircraft," the report said.

Shaheen added that "voluminous documentation demonstrates beyond reasonable doubt" that the State Department approved release of the planes "based solely upon foreign policy considerations."

Shaheen said he reinterviewed Zbigniew Brzezinski, then White House national security affairs adviser, last Jan. 13 on details reflected in his telephone and appointment logs that came to light after an interim report in October.

The logs indicated meetings with Stansfield Turner, then CIA director, in Brzezinski's White House office "on several significant dates" other than a much-discussed session the two men had on March 31, 1980, about an intelligence report concerning Billy Carter.

"Dr. Brzezinski unequivocally stated that those other meetings had no relation to Billy Carter," the report said. "He was also questioned as to the president's involvement, if any, in the meeting with Turner which did involve Billy Carter. Dr. Brzezinski stated that the president had no such involvement."