A video posted online claims to show that Islamic State militants have killed the captured US aid worker Abdul-Rahman Kassig.
In the video, a masked militant stands over a severed head which he says is that of Mr Kassig.
The US says it is working to determine the authenticity of the video, which also shows a mass killing of Syrian troops.
Mr Kassig, also known as Peter, was captured last year.
His family, who live in the US state of Indiana, said they were awaiting confirmation of the reports about their “treasured son” and had nothing more to say at this time.
If his death is confirmed, the 26-year-old will be the fifth Western hostage to have been killed by IS, following the murders of British men Alan Henning and David Haines, and US journalists James Foley and Steven Sotloff.
Analysis: Frank Gardner, BBC security correspondent
This is perhaps the most elaborate and graphic of all the murder videos posted online by IS.
Much of it is taken up with a recent history lesson on Iraq and Syria as seen through the eyes of the jihadists. But the latter part shows a mass beheading of Syrian prisoners in revolting, lingering detail.
Unlike earlier videos, this one revels in gore. Amongst the boiler-suited captives murdered in cold blood is a man IS says is the former US soldier Peter Kassig, who converted to Islam and changed his name to Abdul-Rahman.
Neither his conversion nor the fact that he was helping refugees when he was captured a year ago, appear to have saved him.
Mr Kassig’s murder shows that IS have no intention of toning down their bloodlust to appeal to more followers.
It is also a sign of frustration that they are unable to hit back at the coalition air strikes that have driven them off key sites like the Mosul and Haditha dams, and prevented them from seizing the town of Kobane.