Dzhokhar Tsarnaev, a 19-year-old ethnic Chechen, can be seen in video taken by security cameras placing a backpack near the finish line of the world-renowned race last Monday. He acted in concert with his older brother, who was killed during a shootout with police early Friday.

One of the two perpetrators of the April 15 Boston Marathon bombings is dead and the other is seriously wounded, lying in a hospital bed in police custody. They killed three innocent people and wounded over 170. The question is: Why did they do this?

The older brother, Tamerlan Tsarnaev, 26 was a young man of paradoxes: A reputedly accomplished boxer whose ambition was to make the U.S. Olympic boxing team. He was also a Muslim Jihadist who created a YouTube channel "Terrorists".

The younger brother, Dzhokhar, 19, a graduate of Cambridge Rindge and Latin School, where both my daughters went in the early 80s (their most famous schoolmate was Pat Ewing, who later became the New York Nick's high-scoring center). Dzhokhar was captain of the wrestling team and even won a $2,500 college scholarship. He did poorly at UMass, Dartmouth,

Both brothers were of ethnic Chechen descent. Tamerlan was a legal resident, married an American and had a daughter. Dzhokhar became a naturalized U.S. citizen. We may never know whether they acted independently or as part of a cell, as one of them is dead and the other is seriously wounded and charged with using a weapon of mass destruction that caused death and injury, charges carried a maximum penalty of death.

According to the final report of the Integrated United States Database (IUSSD): Data on the Terrorist Attacks in the United States Homeland, 1970 to 2011, there were a total of 2,608 terrorist attacks in the U.S. between those years.* The deadliest was, of course, the September 11, 2001 Al Qaeda strikes on the World Trade Center towers in New York City, the Pentagon and Shanksville, PA, killing close to 3,000.

The next deadliest terrorist strike was the bombing of the Alfred P. Murrah Federal Building in Oklahoma City on April 19, 1995, which claimed 168 lives, including 18 children and injured more than 680 people. The bomber was a domestic terrorist, Timothy McVeigh, aided by Terry Nichols. McVeigh was a militia movement sympathizer, a Gulf War veteran, motivated by his hatred of the federal government, especially its handling of the Waco Siege of 1993, where 76 members of the Branch Davidian cult died.

Of greater interest to us in connection with the Tsarnaev brothers may be attacks by other Islamic extremists, such as the attack on November 5, 2009 when Nidal Malik Hassan, a U.S. Army medical corps officer, opened fire on fellow soldiers at Fort Hood, killing 13 and wounding 32 others. That was considered the worst terrorist attack after September 11, 2001. Hassan acted alone, perhaps under the influence of the radical Imam Anwar al-Awlaki, who was later assassinated by a U.S. drone in Yemen. Hassan's court martial proceedings are scheduled for May 29, 2013.

There were two failed attacks. On Christmas Day 2009, Umar Farouk Abdulmutallab, a 24-year-old Nigerian man, tried to bomb Northwest Airlines Flight 253 from Amsterdam to Detroit; however the explosives which he carried in his underwear failed to ignite. He was also linked to the radical cleric Anwar al-Awlaki and was trained in Yemen by the Yemen-based Al Qaeda in the Arabian Peninsula. He pleaded guilty and faces life imprisonment.

Faisal Shazhad, a Pakistani-American citizen tried to car bomb New York's Times Square on May Day of 2010; however this bomb also failed to detonate. Shazhad was arrested two days later and sentenced to life imprisonment without possibility of parole. He was trained in bomb making at a camp run by a militant Islamic faction in the Waziristan region in Pakistan and claimed that he was acting in retaliation for U.S. drone attacks. Al Qaeda attacks U.S. targets not because of U.S. values, as George W Bush claimed, but because of U.S. policy.

Most terrorist attacks, however, were carried out by domestic elements. The terrorist organization with the single largest number of attacks in the United States is the Fuerzas Armadas de Liberacion Nacional, a clandestine paramilitary organization which advocates independence for Puerto Rico. It has made 119 attacks causing 7 fatalities. It is followed by the New World Liberation Front (86 attacks, 1 fatality).

It is interesting to note that the Animal Liberation Front and Earth Liberation Front are the third and fourth ranked groups, respectively. The former claims it is not a terrorist organization because it only removes animals from laboratories and farms, arranges safe houses and sanctuaries for animals and harms no one. But it does destroy facilities. It has made 85 attacks in total.

Likewise, the Earth Liberation Front has not physically harmed anyone. But it has set fire to (for example) SUV dealerships, because it believes those vehicles pollute the environment. In all it has made 76 attacks.

There are also vicious killers who murder abortion doctors in the name of righteousness.

So there are all sorts of terrorists who use illegal force and violence to achieve different purposes. According to the FBI, only 6 percent of terrorist attacks in the United States between 1980 and 2005 were committed by Muslims. It seems that Muslims have become the scapegoat in the U.S. whenever terrorism raises its ugly head.

*The US federal code defines terrorism as "the unlawful use of force and violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives."

Global Terrorism Database (GTD) defines terrorism as "the threatened or actual use of illegal force and violence by a non-state actor to attain political, economic, or social goal through fear, coercion or intimidation."

The author is a columnist with China.org.cn. For more information please visit: http://www.ccgp-fushun.com/opinion/zhaojinglun.htm

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