In the Truong and Humphrey case, the federal courts stood behind the executive branch. Appeals brought by lawyers for Mr. Truong and Humphrey, claiming the evidence against them was illegally obtained, were denied.

The courts’ stance galvanized civil libertarians in their campaign for changes in the law. Senator Edward M. Kennedy, a prime sponsor of the 1978 intelligence surveillance law, said, “The recent prosecution against Humphrey and Truong points out the need for this legislation.”

“Serious constitutional issues are raised by the case,” he said in a speech on the Senate floor.

The law created the Foreign Intelligence Surveillance Court, a panel of federal judges known as the FISA court. It mandated that one of those judges review any future requests by the executive branch to employ such techniques in cases concerning national security. (After the terrorist attacks of Sept. 11, 2001, several laws were passed at the behest of the Bush administration to widen the president’s ability to bypass the legal system when conducting wiretaps.)

Mr. Truong and his wife, Carolyn Gates, an American economist, left the United States in the 1980s to live in the Netherlands, where both worked as economic development consultants to the European Commission, the executive branch of the European Union.

Mr. Truong had come to the United States in 1965 as a student, and earned degrees in political science and economics at Stanford University. Discouraged by his family from returning home — he was the son of an opposition leader jailed in Saigon by South Vietnam’s American-backed government — Mr. Truong became an outspoken antiwar activist in the United States.

When he was arrested on Jan. 31, 1978, Mr. Truong was doing postgraduate work at American University, and working as a lobbyist in Washington for several organizations advocating normalization of diplomatic ties between the United States and his country, which had reunified under Communist rule since South Vietnam’s collapse in 1975.

Basing their allegations on wiretaps and the testimony of a double agent employed by both the C.I.A. and the F.B.I., Justice Department prosecutors said Mr. Truong had been passing secret State Department cables about Vietnam-related foreign policy strategies to Vietnamese agents in Paris. Diplomats from the two countries were in negotiations there over an accounting of American prisoners of war and soldiers missing in action as well as other matters left unresolved at the end of the war.