WASHINGTON - A U.S. military drone strike in Yemen last December may have killed up to a dozen civilians on their way to a wedding and injured others, including the bride, a human rights group says. U.S. officials say only members of al-Qaida were killed, but they have refused to make public the details of two U.S. investigations into the incident.

Human Rights Watch released a report on the drone strike Thursday, citing interviews with eight witnesses and relatives of the dead as well as Yemeni officials. The report said four Hellfire missiles were fired at a wedding procession of 11 vehicles on Dec. 12, 2013, in Radda in southern Yemen, killing at least 12 men and wounding at least 15 others, six of them seriously.

The report said the procession "may have included members" of Yemen's al-Qaida in the Arabian Peninsula, "although it is not clear who they were or what was their fate." Family members and survivors say all those hit were civilians; Yemeni officials told Human Rights Watch that most were militants.

"We asked both the Yemeni and the U.S. authorities to tell us which of the dead and wounded were members of militant groups and which if any were civilians," report author Letta Tayler, a senior terrorism and counterterrorism researcher at Human Rights Watch, told The Associated Press. "They did not reply to this question."

She added: "While we do not rule out the possibility that AQAP fighters were killed and wounded in this strike, we also do not rule out the possibility that all of those killed and wounded were civilians."

The New York-based group called on the U.S. government to investigate and make the findings public.

A Pentagon spokesman, Navy Rear Adm. John Kirby, said he would not comment on specific operational details. He noted that the Yemeni government has stated that the targets were "dangerous senior al-Qaida militants."

U.S. and Yemeni officials said the target of the attack, Shawqi Ali Ahmad al-Badani, a midlevel al-Qaida leader, was wounded and had escaped.

Al-Badani is on Yemen's most wanted list and is accused of masterminding a plan for a major attack last summer. When an intercepted message revealed the plot, the U.S. temporarily closed 19 of its diplomatic posts across Africa and the Mideast. Some European missions were closed as well.

Three U.S. officials said the U.S. government did investigate the strike against al-Badani - twice - and concluded that only members of al-Qaida were killed in the three vehicles that were hit.

The officials say the Pentagon can't release details, because both the U.S. military and the CIA fly drones over Yemen. By statute, the military strikes can be acknowledged, but the CIA operations cannot. The officials said if they explain one strike but not another, they are revealing by default which ones are being carried out by the CIA.

The officials spoke on condition of anonymity because they were not authorized to discuss the drone operations publicly.

The continued secrecy surrounding the drone program shows how the Obama administration has been slow to transfer the CIA drones over to the military's Joint Special Operations Command nearly a year after Obama promised in a May 2013 speech to put the military largely in charge of lethal strikes and thereby make the program more transparent. Congress has objected to the transfer to the military, because the CIA can strike in countries where the military cannot - for instance, in countries that refuse to allow U.S. counterterrorist actions on its soil.

With the drone program in limbo, U.S. officials have simply continued to say nothing of the strikes, wherever they occur.

"The U.S. refusal to explain a deadly attack on a marriage procession raises critical questions about the administration's compliance with its own targeted killing policy," Tayler wrote in the report.

Obama's new guidelines include using lethal force "only to prevent or stop attacks against U.S. persons, and even then, only when capture is not feasible and no other reasonable alternatives exist to address the threat effectively." It also requires "near certainty" of no civilian casualties.

"When we believe that civilians may have been killed, we investigate thoroughly," said Caitlin Hayden, a National Security Council spokeswoman. "In situations where we have concluded that civilians have been killed, the U.S. has made condolence payments where appropriate and possible." She would not say whether the U.S. contributed any money to families of the dead in this incident.

The local Yemeni governor and military commander called the strike a mistake and compensated the families of those killed and injured.

Lt. Gen. Joseph Votel, commander of Joint Special Operations Command, ordered an independent investigation by an Air Force general and the White House requested another by the National Counterterrorism Center. Both concluded no civilians were killed. Votel's staff also showed lawmakers video of the operation. Two U.S. officials who watched the video and were briefed on the investigations said it showed three trucks in the convoy were hit, all carrying armed men.

The Human Rights Watch report lists the names and ages of 12 men who witnesses said were killed in the attack, along with the names of six men who were seriously wounded.

The strikes are part of a joint U.S.-Yemeni campaign against AQAP, considered the most dangerous al-Qaida branch. The group is blamed for a number of unsuccessful bomb plots aimed at Americans, including an attempt to bring down a U.S.-bound airliner with explosive hidden in the bomber's underwear and a second plot to send mail bombs hidden in the toner cartridges on planes headed to the U.S. The group also aspires to govern in Yemen, and it seized large areas of the south before the military drove most of them out in the summer of 2012.

According to the nonpartisan public policy institute New America Foundation, the U.S. has launched 99 drone strikes in Yemen since 2002.