The UK's security and intelligence agencies are examining a video by so-called Islamic State, to identify a man and young boy with British accents.
The video purportedly shows the killing of five men who IS says were spying for the UK.
The authorities will also be carrying out voice-print analysis to match what is heard against individuals known to have travelled to Iraq and Syria.
The man threatens attacks in the UK, during the 10-minute film.
He also says the footage, which has not been independently verified, is a message for Prime Minister David Cameron.
After the apparent killings the young boy, who seems to be aged about six or seven years old and is wearing military-style clothing, is seen pointing into the distance and talking about killing "unbelievers".