They are called "homegrown terrorists," Western citizens highly prized by Islamic militant groups because they can move across borders and carry out attacks easier than people from Middle East or South Asian nations.

Two such people — one Canadian and one Australian — are believed to have been involved in the July 18 bus bombing in Bulgaria that killed five Israeli tourists and a Bulgarian driver, according to Bulgarian investigators. Interior Minister Tsvetan Tsvetanov of Bulgaria said the two were members of the Lebanese Shiite Muslim militant group Hezbollah, which in turn is linked to Iran.

Here are some examples of Western citizens who have been linked to terrorism both in their home countries or abroad in recent years:

LONDON SUBWAY BOMBING

Four young Britons — three of Pakistani and one of Jamaican origin — carried out a series of suicide attacks July 7, 2005, on the London public transport that killed 56 people. More than 700 people were injured. All four had lived normal lives under the police radar and had no criminal records. They carried home-made bombs in backpacks. Al-Qaida released video testimonies of two of the bombers who denounced the West and declared their allegiance to Osama bin Laden.

SHOE BOMBER

Richard Reid was a British citizen who converted to Islam in prison. After his release he traveled to Afghanistan and Pakistan, where authorities say he trained with al-Qaida. More than three months after Sept. 11 attacks, Reid boarded an American Airlines flight in Paris bound for Miami and tried to detonate a bomb in his shoes. He was subdued by passengers and crew members, and the plane landed safely in Boston. In 2002 Reid was sentenced to life without parole after pleading guilty to eight counts of terrorism and attempting to destroy a commercial airliner.

DAVID COLEMAN HEADLEY

Headley, a Pakistani-American, used his U.S. passport to travel frequently to India, where he allegedly scouted out venues for terror attacks on behalf of the Lashkar-e-Taiba terrorist organization. The al-Qaida-affiliated group used the information to plan and carry out the 2008 attacks in Mumbai, India, in which more than 160 people died. Last month Headley was sentenced by a U.S. federal court in Chicago to 35 years in prison for his role in the Mumbai attacks.

TIMES SQUARE FAILED BOMBING

On May 1, 2010, two street vendors alerted police to smoke coming out of a vehicle parked on New York's Time Square — an area teeming with tourists. Police found the vehicle was rigged with a bomb that failed to explode. Two days later, federal agents in New York arrested Faisal Shahzad, 30, a Pakistan-born U.S. citizen who lived in Bridgeport, Connecticut, after he had boarded a flight bound for Dubai in the Persian Gulf. Shahzad confessed to the attempted car bombing and said he had trained at a Pakistani terror training camp. Shahzad was sentenced to life imprisonment in October 2010.

ANWAR AL-AWLAKI

Al-Awlaki was born in 1971 in New Mexico, where his father was studying agriculture as a Fulbright scholar. The son was educated in the United States but left in 2002, eventually returning to Yemen where he became a key figure in the local al-Qaida branch, which U.S. authorities believed was the most dangerous of the al-Qaida franchises. Al-Awlaki's fluent English and articulate speaking style won him a huge following among disaffected young Muslims in the West. He and another American, Samir Khan, who edited al-Qaida's Internet magazine, were killed in a U.S. drone attack in Yemen on Sept. 30, 2011. Continued...