Terrorist groups such as al Qaeda and ISIS are causing death and destruction around the world. This week’s attacks in Canada again draw attention to radical Islamists.

The solitary fanatics – the ‘lone wolves’ – present law enforcement with difficult and frightening challenges. These domestic terrorists  are not tied to any formal cells. They often become radicalized over the Internet, in prison and may live undetected within our communities – until they strike.

"Greta Investigates" digs deep into the 2014 crimes of Ali Muhammad Brown, now sitting in a New Jersey jail, after he admitted to murdering four people in a cross-country killing spree he says he committed in the name of jihad.

He claims he killed the four men as “vengeance” for the U.S.’s military action overseas. Were these murders acts of terror on U.S. soil? 

Greta will take you through his direct and disturbing ties to a "jihadi" barbershop in Seattle which was at the center of a federal terrorism investigation in the mid 2000s. 

In this Fox News special, we examine other cases of ‘lone wolf’ terrorism in the United States and the UK, and explore the attacks in Canada which are following a familiar pattern of radicalization.

We also examine Nidal Hasan’s rampage at Fort Hood and the female radical Roshonara Choudhry, who stabbed a British government official. Both Hasan and Choudhry were followers of the radical American-born cleric Anwar Awlaki.

See it all on 'Greta Investigates: The Lone Wolves of Terror,' Friday at 7p/2a ET, Saturday at 10p/1a ET and Sunday at 9p/12a ET.\

Watch a preview clip: