The FBI has identified the masked executioner in the Islamic State beheading videos of U.S. journalists Steven Sotloff and James Foley and British hostage David Haines. FBI Chief James Comey declined Thursday to name the militant or provide further details, according to ABC News. The gruesome videos helped rally public sentiment against the extremist group, also known as ISIS and ISIL, before Obama began airstrikes in Iraq and Syria against the militants in recent weeks.

The release of the video purporting to show Foley's death in August was the first in the series of high-production messages aimed at Western leaders. In each video, a masked executioner with an apparent British accent began the beheading process before issuing a threat to kill another Westerner if airstrikes against ISIS continued. ISIS has threatened to kill British aid worker Alan Henning next. 

Foley was a freelance journalist, originally from New Hampshire, who wrote for media outlets including Global Post. New Hampshire Gov. Maggie Hassan and U.S. Sens. Jeanne Shaheen and Kelly Ayotte were among the hundreds who attended his funeral mass in August. 

ISIS captured Sotloff near the Syria-Turkey border in August 2013, when he was working for Time magazine. The Haines video, the most recent, was released on Sept. 13. The 44-year-old aid worker was kidnapped in March 2013.

The U.S. began airstrikes in Syria against ISIS and other militants group Monday. Five Arab states are supporting the U.S., while France is striking against the Islamic State group in Iraq at the request of the Iraqi government. The Obama administration launched similar strikes in Iraq against ISIS in August.

President Barack Obama vowed to go after Islamic State group militants wherever they may be in Sept. 10 speech after the beheading videos were made public.

"In a region that has known so much bloodshed, these terrorists are unique in their brutality. They execute captured prisoners. They kill children. They enslave, rape and force women into marriage. They threatened a religious minority with genocide. In acts of barbarism, they took the lives of two American journalists – Jim Foley and Steven Sotloff," Obama said at the time.