WASHINGTON — The Supreme Court ruled unanimously on Wednesday that the family of an American citizen killed during a visit to the West Bank may not sue the Palestinian Authority and the Palestine Liberation Organization under a 1991 federal law, the Torture Victim Protection Act.

The law allows civil lawsuits against “an individual” who engages in torture or killings, and the question in the case was whether that term could apply not only to people but also to organizations.

Justice Sonia Sotomayor, writing for the court, said the answer was no.

“An officer who gives an order to torture or kill is an ‘individual’ in that word’s ordinary usage; an organization is not,” she wrote.

The case was brought by the family of Azzam Rahim, a naturalized American citizen. According to their lawsuit, Mr. Rahim was arrested by intelligence officers of the Palestinian Authority during a 1995 visit to the West Bank. The officers took him to a prison in Jericho, where he was tortured and killed, the lawsuit said.