The disclosure was made in the case of Deborah Peterson, the sister of James Knipple, a U.S. Marine killed when terrorists bombed the barracks of American armed services personnel in Beirut in October 1983. Victims of terrorist attacks and their survivors have sued banks including Bank Markazi Jomhouri Islami Iran in Manhattan federal court to enforce collection of a $2.7 billion judgment.

The probe also includes an investigation of obstruction and illegal monetary transactions, and violations of the International Emergency Economic Powers Act, Assistant U.S. Attorney Alexander Wilson said in the subpoena.

Clearstream, a bank based in Luxembourg, agreed in January to pay $152 million to settle civil claims it violated U.S. economic sanctions on Iran. The bank used an account at an unidentified U.S. financial firm to hold $2.81 billion in securities on behalf of the Central Bank of Iran, according to the Treasury Department.

The accord didn’t bar the U.S. from prosecuting the bank, according to the agreement.