LONDON—The masked Islamic State militant known as Jihadi John, who has appeared in several videos showing the beheading of Western hostages, has been identified as British citizen Mohammed Emwazi, Western officials said.

Mr. Emwazi, who is in his 20s and was born in Kuwait, grew up in London and attended university here, according to officials and a person familiar with the situation. He appears to have been on the radar of British officials since at least 2009, when they believed he was trying to travel to Somalia for training with Islamist extremists—which he denied—according to correspondence between Mr. Emwazi and a civil-rights group he interacted with.

Western officials said Mr. Emwazi traveled to Syria in 2012, later joining Islamic State, and is believed to be moving around the region.