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.