BEIRUT (AP) — An Islamic State group video released Sunday purports to show extremists beheading a dozen Syrian soldiers and ends with a militant claiming to have killed U.S. aid worker Peter Kassig, the latest slaughter proudly broadcast by the group on the Internet.

The video ends with the militant standing over a severed head he says belongs to Kassig. U.S. officials said they were working to determine the video’s authenticity. Kassig’s family said it was awaiting the outcome of the investigation.

“We prefer our son is written about and remembered for his important work and the love he shared with friends and family, not in the manner the hostage takers would use to manipulate Americans and further their cause,” the family said in a statement.

The Associated Press could not independently verify the footage, though it appeared on websites used in the past by the Islamic State group, which now controls a third of Syria and Iraq.

The video identifies the militants’ location as Dabiq, a town in northern Syria that the Islamic State group uses as the title of its English-language propaganda magazine and where they believe an apocalyptic battle between Muslims and their enemies will take place.

The slickly produced video shows the beheadings of about a dozen men identified as Syrian military officers and pilots, all dressed in blue jumpsuits. The main militant in the video who speaks to the camera has a British accent and warns that U.S. soldiers will meet a similar fate.

“We say to you, Obama: ... You claim to have withdrawn from Iraq four years ago,” the militant said. “Here you are: You have not withdrawn. Rather, you hid some of your forces behind your proxies.” A U.S.-led coalition is targeting the Islamic State group in airstrikes, supporting Western-backed Syrian rebels, Kurdish fighters and the Iraqi military.

Audio in the video appears to have been distorted to make it more difficult to identify the militant. Previous videos featured a militant with a British accent that the FBI says it has identified, though it hasn’t named him publicly.

Later, the militant claims Kassig was killed because he “fought against the Muslims in Iraq, while serving as a soldier.” Kassig, from Indianapolis, Indiana, served in the U.S. Army’s 75th Ranger Regiment, a special operations unit, and deployed to Iraq in 2007.

After being medically discharged, Kassig formed the aid organization Special Emergency Response and Assistance, or SERA, in Turkey to aid Syrian refugees. He began delivering food and medical supplies to Syrian refugee camps in 2012 and provided trauma care to wounded Syrian civilians. Friends say he converted to Islam in captivity and took the first name Abdul-Rahman.

Burhan Agha, a Syrian friend who worked with Kassig in Lebanon and who moved to Switzerland seeking asylum, wept when recounting his generosity Sunday.

“If I could apologize to each American, one by one, I would, because Peter died in Syria, while he was helping the Syrian people,” Agha told the AP by telephone. “Those who killed him claimed to have done it in the name of Islam. I am a Muslim and am from Syria. ... (His killers) are not Muslims.”

The White House said the U.S. intelligence community was working to determine the authenticity of the video. National Security Council spokeswoman Bernadette Meehan said that if the video is authentic, the White House would be “appalled by the brutal murder of an innocent American.”

Previous videos have shown the beheading of two American journalists and two British aid workers. The latest video did not show the person identified as Kassig being beheaded. Unlike previous videos, it did not show other Western captives or directly threaten to behead anyone else.

The Islamic State group also holds British photojournalist John Cantlie, who has appeared in several videos delivering statements for the group, likely under duress.

The Islamic State group has beheaded and shot dead hundreds of captives — mainly Syrian and Iraqi soldiers — during its sweep across the two countries, and has celebrated its mass killings in high-quality, extremely graphic videos.

The group has declared a self-styled Islamic caliphate in the areas under its control, which it governs according to its violent interpretation of Shariah law.

The U.S.-led coalition later began its campaign of airstrikes in Iraq and Syria in a bid to halt the group’s rapid advance and eventually degrade and destroy it.

The Islamic State group has its roots in al-Qaida’s Iraqi affiliate but was expelled from the global terror network over its brutal tactics and refusal to obey orders to confine its activities to Iraq. It became even more extreme amid the bloody civil war in neighboring Syria, growing stronger to the point of being able to launch a lightning offensive across Iraq.

The fight against the militant group adds another layer to Syria’s complex civil war, now in its fourth year, which began as an uprising against President Bashar Assad. Activists say that conflict has killed more than 200,000 people.

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Associated Press writers Julie Pace in Brisbane, Australia, David Aguilar in Detroit and Jon Gambrell in Cairo contributed to this report.