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.

MAJ. NIDAL MALIK HASAN

Born in Arlington, Virginia, to Palestinian parents, Hasan joined the U.S. Army in college and became a military psychiatrist. Colleagues said during an assignment at Walter Reed Medical Center, he was deeply affected by dealing with young soldiers wounded in Iraq and Afghanistan. FBI investigators alleged that he corresponded by email with al-Awlaki. Hasan was wounded and captured by police on Nov. 5, 2009, after he allegedly opened fire on soldiers in Fort Hood, Texas, killing 13 and wounding more than two dozen. Hasan, who was paralyzed from the waist down in the shooting, was charged with 13 counts of premeditated murder and 32 counts of attempted premeditated murder. A trial date has not been set, and he could face the death penalty if convicted.

ADAM GADAHN

Born Adam Pearlman in Oregon, Gadahn converted to Islam in 1995 and moved to Pakistan, where he joined al-Qaida as a propagandist. Using the name "Azzam the American," he appeared in numerous al-Qaida videos, denouncing U.S. moves in Afghanistan and elsewhere and threatening attacks on Western interests abroad. U.S. authorities filed treason charges against him in 2006 and have offered a $1 million reward for information leading to his arrest or conviction. Despite rumors he had been killed or captured, Gadahn appeared in a video last September marking the 11th anniversary of the Sept. 11 attacks.

GLASGOW AIRPORT ATTACK

On June 30, 2007, a jeep loaded with propane canisters slammed into the terminal of the Glasgow International Airport in Scotland, setting the building on fire. Five bystanders were injured. Both occupants of the vehicle were arrested. Police identified them as Bilal Abdulla, a British-born, Muslim doctor of Iraqi descent and Kafeel Ahmed, the driver. Anti-terrorism officials said Abdulla became radicalized due to the Iraq war. Ahmed, an Indian engineering student, died of his burns. Abdulla was convicted of conspiracy to commit murder and was sentenced to 32 years in prison.

AHMAD OMAR SAEED SHEIKH

Following his education in Britain, the British-born Sheikh traveled to South Asia, where he joined Islamic militant groups. He was sent to prison for kidnapping Western tourists in India in 1994, but was released to Pakistan five years later in an exchange of prisoners following the hijacking of an Indian airliner to Afghanistan. In 2002 he was convicted of kidnapping and murder in the death of Wall Street Journal reporter Daniel Pearl and sentenced to death. His appeal is still pending in a Pakistani court.