Family members of three U.S. citizens killed last year in drone strikes in Yemen recently filed a lawsuit here accusing U.S. intelligence and military officials of violating the victims’ rights under the U.S. constitution and international law.

Prepared by the American Civil Liberties Union and the Center for Constitutional Rights, the lawsuit marks a major legal challenge to the U.S. policy of extrajudicial “targeted killings” of suspected terrorists far from traditional battlefields, such as Afghanistan.

“These killings rely on vague legal standards, a closed executive process, and evidence never presented to the courts,” according to the 17-page complaint, which noted that the practice has “resulted in the deaths of thousands of people, including many hundreds of civilian bystanders,” in Yemen, Somalia, Pakistan, Sudan, and the Philippines since 2001...