A U.S. drone strike in December that killed at least a dozen people in Yemen failed to comply with rules imposed by President Obama last year to protect civilians, according to an investigation by a human rights organization released Thursday.

The report by Human Rights Watch concluded that the strike, which was carried out by the U.S. military’s Joint Special Operations Command, targeted a line of vehicles that were part of a wedding procession, and that evidence indicates “some, if not all those killed and wounded were civilians.”

The findings contradict assertions by U.S. officials that only militants were killed in the operation, although the report acknowledged that members of al-Qaeda in the Arabian Peninsula, the terrorist network’s affiliate in Yemen, may have been among the dead.

Overall, Human Rights Watch “found that the operations did not comply with the targeted killing policies that President Obama outlined” in a speech in May, the report said, citing in particular Obama’s requirement of “near-certainty” that no civilians would be harmed.

The report represents the most detailed independent examination to date of a strike that has focused attention on the administration’s struggles to tighten the rules for targeted killing, provide more information about such operations to the public and gradually shift full control of the drone campaign from the CIA to the Pentagon.

1 of 7 Full Screen Autoplay Close Skip Ad × U.S. drone strikes stirring outrage in Yemen View Photos As the strikes continue, public outrage is rising in Yemen, where many people, including government officials, argue that the attacks increase sympathy for al-Qaeda. Caption As the strikes continue, public outrage is rising in Yemen, where many people, including government officials, argue that the attacks increase sympathy for al-Qaeda. Jan. 17, 2014 Politicians, officials and intellectuals gather to talk politics and chew khat, a leafy narcotic, in the home of a prominent Yemeni politician in Sanaa, Yemen. Local anger has centered on the U.S. government’s highly secretive drone programs here. The U.S. drone campaign in Yemen is aimed at rooting out al-Qaeda in the Arabian Peninsula, which U.S. officials have called the most active and deadly of the organization’s wings. Drones have carried out at least 80 attacks since the start of 2011, according to the Long War Journal, which tracks U.S. counterterrorism efforts. Abigail Hauslohner/The Washington Post Buy Photo Wait 1 second to continue.

Caitlin Hayden, a spokeswoman for the National Security Council, declined to comment on the report or the Dec. 12 strike but said the United States takes “extraordinary care” in its counter­terrorism operations to avoid civilian casualties and noted that Yemeni officials described the targets as “dangerous senior al-Qaeda militants.”

The investigation by Human Rights Watch, a group that has been consistently critical of the targeted killing program, concluded that the attack killed 12 men, ages 20 to 65, and wounded 15 others, citing accounts from survivors, relatives of the dead, local officials and news media reports.

The attack targeted a convoy of 11 vehicles traveling from the site of a wedding near the city of Rad’a to the groom’s village, according to the report, which said that the bride was among a small number of female travelers and that “shrapnel grazed the bride under one eye, and blew her trousseau to pieces.”

Investigators said they spoke with senior Yemeni officials including President Abed Rabbo Mansour Hadi, as well as U.S. officials in Washington. The report also includes photos of metal shards identified as shrapnel from four Hellfire missiles that were fired.

U.S. officials have said that the main target of the strike was Shawqi Ali Ahmed al-Badani, an alleged al-Qaeda operative accused of leading a plot that prompted the United States to close dozens of diplomatic facilities in the Middle East last year.

Neither Badani’s name nor that of any other senior al-Qaeda operative appears on a list of those killed or wounded that is included in the report. But Letta Tayler, the lead investigator for Human Rights Watch, said she could not rule out that some of those killed were associated with al-Qaeda.

Accounts from Yemeni officials “have shifted over time,” according to the report, which described a meeting in Rad’a two days after the strike in which a senior Yemeni military officer and the governor of the province publicly apologized and called the attack a “mistake.”

Participants in that meeting told Human Rights Watch that Yemeni officials distributed cash totaling about $159,000 as well as 101 Kalashnikov assault rifles to relatives of those killed or wounded, payments described in the report as “a tribal gesture of apology.”

Although the CIA also operates armed drones in Yemen, U.S. officials have said that the Dec. 12 strike was carried out by the JSOC. Obama’s plan to shift the operation to the Pentagon has drawn opposition on Capitol Hill, where senior lawmakers — including Sen. Dianne Feinstein (D-Calif.), the chairman of the Senate Intelligence Committee — have questioned whether the military can match the CIA’s effectiveness and avoid civilian casualties.

Because of the high casualty count and number of vehicles involved, the U.S. military launched an internal investigation of the December strike. U.S. officials briefed on that investigation said it supports the U.S. contention that the targets were al-Qaeda militants, but they declined to elaborate. A Pentagon spokesman declined to comment.

Human Rights Watch said its efforts to get access to the military’s findings or other U.S. information on the attack were rebuffed. “The U.S. government has provided no information as to its legal basis for attacking the wedding procession,” the report said.