While the 1976 Foreign Sovereign Immunities Act broadly protects foreign governments against lawsuits filed by U.S. citizens, the act provides exemptions, and federal judges have allowed suits to proceed in two notorious cases involving state-sanctioned assassinations.

In one, the survivors of Orlando Letelier, former Chilean ambassador to the United States, were allowed to sue the Chilean government after it was established in criminal proceedings that Letelier had been killed in a Washington bombing involving four operatives and senior officials of the Chilean intelligence services and two Cuban exiles.

In the other, the widow of Henry Liu, a Chinese journalist and critic of the Taiwan government, was allowed to sue Taiwan after a federal court ruled that her husband had been slain in California by two Chinese gang members acting for Adm. Wong Hsi-ling, former director of Taiwan's Defense Intelligence Bureau.

A federal judge in Washington found in the Letelier case that Chile did not qualify for immunity because the assassination plot had been carried out by intelligence operatives targeting critics of Chilean dictator Augusto Pinochet.

In his ruling, the judge said a foreign government "has no discretion to perpetrate . . . action that is clearly contrary to the precepts of humanity as recognized in both national and international law."

In the Liu case, the U.S. Court of Appeals for the Ninth Circuit reached a similar conclusion, setting a high - but not insurmountable - bar for suing foreign governments in cases of egregious government conduct.

While the Foreign Sovereign Immunities Act provides foreign governments with blanket immunity from lawsuits filed by the U.S. government, it permits suits filed by private parties under certain circumstances and one significant condition:

Plaintiffs must show that whatever harm was caused was the result of criminal behavior or some other action outside the boundaries of normal government operations.

Once that high hurdle is cleared, U.S. citizens can sue under circumstances, including:

Cases of personal injury or death, or damage to or loss of property, occurring in the United States if caused by negligent acts of omission or commission by a foreign government.

Cases of personal injury or death caused by an act of torture, extrajudicial killing, aircraft sabotage, hostage taking, or the provision of material support or resources for such an act, if the foreign state is designated as a state sponsor of terrorism.

Such was the case with Libya, which sponsored the bombing of Pan Am Flight 103 over Lockerbie, Scotland, on Dec. 21, 1988, killing 270 people.

When a relative of one victim first sued Libya, a federal court in New York threw out the suit, citing the Foreign Sovereign Immunities Act. Families of Pan Am victims then lobbied Congress and won passage of the exemption citing aircraft sabotage and other criminal acts.

With that provision of the law in place, the families sued again and won. Libya negotiated a $2.7 billion settlement, most of which has been paid out.