British spy agencies are facing questions about how a young Londoner who was on their radar as part of terrorist investigations was able to travel to Syria and become the knife-wielding masked militant known as "Jihadi John."

Officials have identified the man shown in hostage-beheading videos as Mohammed Emwazi, a Kuwait-born computer science graduate raised and educated in Britain.

Emwazi, now in his mid-20s, was known to the British intelligence services since 2009, in connection with investigations into Islamic terrorism in Somalia and elsewhere.

Prime Minister David Cameron on Friday defended the security services, saying they are doing an "incredibly impressive" job keeping the country safe.

"All of the time, they are having to make incredibly difficult judgments and I think basically they make very good judgments on our behalf," he said in Wales.

He did not mention Emwazi by name but said it was his top priority "to find these people and put them out of action." He said anyone who commits "appalling and heinous" crimes against British citizens anywhere in the world will be tracked down.

The failure to prevent Emwazi from traveling to Syria to join extremists has highlighted the challenge that intelligence agencies across Europe face as the number of would-be jihadis grows.

Emwazi is one of a number of men from West London believed to have traveled to Syria in 2012. Several are now dead.

Chris Phillips, former head of Britain's National Counterterrorism Security Office, said Friday the case showed that police and intelligence agencies lacked the resources to monitor a growing number of suspects. He said the current control systems are not working and the number of people being radicalized via the Internet is increasing.

The widow of a British aid worker killed by Islamic State militants said Friday she would like to see "Jihadi John" captured and put on trial.

Dragana Haines told The Associated Press in a phone interview from her home in Croatia that "I really hope he will be caught, I think it would be a good lesson for all."

Haines, whose husband was killed in September, said she thinks it would be preferable for him to end up being judged in a court of law because "people of his kind believe that death in combat is an honor, something special."

At the University of Westminster, where Emwazi studied computer science, administrators told students if they had any "concerns" about the news of his involvement in jihad they should call an advice line.