A Human Rights Watch report says a U.S. military drone strike in Yemen last December may have killed up to a dozen civilians on their way to a wedding.

The report, relying on witnesses, relatives and Yemeni officials, says four missiles hit a wedding procession on Dec. 12 in Radda, killing 12 men and wounding some 15 others. Family members say they were civilians, but Yemeni officials say most were militants.

Three U.S. officials say only members of al-Qaida were killed, but they have refused make public the details of two U.S. investigations into the incident. The officials spoke on condition of anonymity because they were not authorized to discuss the drone operations publicly.

New York-based Human Rights Watch released the report Thursday.