The United States considers such strikes against specific militant leaders to be lawful acts of war or self-defense, especially when it is not feasible to attempt to capture the militants but they can be struck by a missile fired from a drone or warplane.

In 2014, when the United States began its air campaign against the Islamic State, the Obama administration declared the operations to be part of the existing armed conflict that Congress authorized against Al Qaeda after the terrorist attacks of Sept. 11, 2001. The group now known as the Islamic State was a component or affiliate of Al Qaeda for years, but it split away over internal disagreements.

Mr. Emwazi, who was first known only as an unidentified, masked man with a British accent, first came to prominence in August 2014, when the Islamic State released a video in which the journalist James Foley was shown reading a statement criticizing President Obama and the American military operation against the Islamic State in Iraq. His captor then beheaded him off camera and then threatened to behead another journalist, Steven J. Sotloff, if his demands were not met.

Two weeks later, the Islamic State released a video showing the masked man beheading Mr. Sotloff.

The Washington Post revealed Mr. Emwazi’s identity in February, reporting that he grew up in a well-off family that moved to Britain when he was a child, and that he had studied computer science at the University of Westminster. The revelation touched off intense examination of the causes of radicalization among Muslim immigrants in Europe.

Mr. Emwazi was part of a group of friends, called the “North London Boys” by some intelligence analysts, who prayed at the same mosque and became captivated by an Egyptian-born cleric, Hani al-Sibai. Mr. Sibai is thought to have close links to the Tunisian branch of Ansar al-Shariah, a Salafist group that has been linked to a deadly attack in June on tourists in Tunisia.

The leader of this network was Bilal al-Berjawi, who was stripped of his British citizenship in 2011 after he went to Somalia to join the Islamist group known as the Shabab, and was killed by an American drone strike the next year. That same year, Mohamed Sakr, another friend, was also killed by a drone strike in Somalia.