A Brief History of Terrorism in the United States

A man carries newspapers featuring the Boston marathon blasts on the front page, while waiting to cross a road in central London, Tuesday, April 16, 2013. (AP Photo/Sang Tan) (National Journal)

Terror in the United States knows many origins. There are "homegrown" incidents such as the 1995 Oklahoma City bombing and the 1996 Atlanta Olympics bombing, in which anti-government extremists used violence to promote their cause. There's also the terror that comes from abroad, as we know all too well the consequences of Islamic extremism.

We don't know why the explosions were set off in Boston yesterday. We don't know if it was perpetrated by a group with an agenda or an individual with none. We won't speculate about it either until the facts are uncovered. But as of now, it's passing the sniff test: Defense Secretary Chuck Hagel today called the attacks "a cruel act of terror."

For all the focus on terrorism in the last decade, it's a word that evades an easy definition. A 2011 report from the Council of Foreign Relations finds several definitions of terrorism that exist within the agencies of the federal government. While some incidents are easily labeled as terror — such as 9/11 — the label becomes harder to stick on cases such as the 2009 Fort Hood shooting. According to the report, lone-wolf violence is on the rise, but "it is difficult for authorities to determine whether such an act of violence falls under the legal definition of "˜domestic terrorism.'"

Over the last two decades, there have been many events that are considered acts of terrorism (officially or not), or potential acts of terrorism. Here are some of the more notable instances:

1978-1995 — The Unabomber

Ted Kaczynski, a recluse ideologically opposed to technological progress, sends 16 bombs through the mail over the course of two decades.

1993 — The First World Trade Center Bombing

Ramzi Yousef, a member of al Qaeda and nephew of Khalid Sheikh Mohammed, loaded a truck bomb into the parking garage below Tower 1 of the World Trade Center. He intended for the bomb to bring down the building. It didn't, but it killed six people and injured 1,042. He later wrote to the New York Times, "This action was done in response for the American political, economical, and military support to Israel, the state of terrorism, and to the rest of the dictator countries in the region."

1995 — Oklahoma City Bombing

Timothy McVeigh and Terry Nichols killed 168 and injured more than 600 people when he exploded a car bomb in the Alfred P. Murrah Federal Building in Oklahoma City. McVeigh wanted revenge over the way the government handled the Waco siege in 1993, and thought the destruction of one federal building could spark a revolution. He was sentenced to death.

1996 — Centennial Olympic Park Bombing

At the 1996 summer Olympics in Atlanta, Georgia, Eric Robert Rudolph planted pipe bombs and shrapnel underneath benches in the main square of the Olympic grounds. Two people died; 111 were injured. "Even though the conception and purpose of the so-called Olympic movement is to promote the values of global socialism," Rudolph said in his confession, "as perfectly expressed in the song Imagine by John Lennon, which was the theme of the 1996 Games even though the purpose of the Olympics is to promote these ideals, the purpose of the attack on July 27 was to confound, anger and embarrass the Washington government in the eyes of the world for its abominable sanctioning of abortion on demand."

2001 — 9/11

The deadliest terrorist attack in U.S. history. Nearly 3000 people died.

2001 — The Shoe Bomber

Richard Reid, a British citizen with ties to Al Qaeda, attempted to blow up a plane with explosives implanted in his shoes. After trying to light a fuse to ignite the explosive, Reid was subdued by other passengers.

2001 — Anthrax Attacks

Biologist Bruce E. Ivins mailed letters laced with anthrax to several media organizations and senator's offices. Five people died and 17 others were infected by the lethal bacterium. According to the FBI,Ivins conducted the attacks to bring attention to an anthrax vaccine program he had worked on. While the attacks were conducted in 2001, Ivins wasn't identified as a suspect until 2008.

2002 — The Beltway Sniper

John Allen Muhammad and Lee Boyd Malvo terrorize the D.C.-beltway area, taking shots at people from the modified trunk of a Chevrolet Caprice sedan. Ten people were killed over the course of three weeks. Three others were injured.

2006 — SUV Attack at University of North Carolina Chapel Hill

Mohammed Reza Taheri-azar, wanting to "avenge the deaths of Muslims worldwide," intentionally hits nine people with an SUV at the University of North Carolina. None died.

2009 — NYC Subway Bomb Plot

Najibullah Zazi, an Afghan immigrant who claimed ties to al Qaeda, was arrested, charged, andadmitted to plotting to plant a bomb in the the New York City subway system. Zazi was constructing explosive materials similar to those used in the 2005 London subway attacks. Mayor Bloomberg characterized it as "a very serious plot against New York City."

2009 — Fort Hood Shooting

Nidal Malik Hasan, a 39-year-old Army psychiatrist, killed 13 and injured 30 in a shooting at an Army base in Fort Hood, Texas. While it was reported Hasan had correspondences with Anwar al-Awlaki — the American terrorist who would later be killed in a drone strike — the shooting has not been classified as a terrorist incident, but an act of workplace violence. As recently as February, Republican lawmakers have called on the administration to label it a terrorist attack.

2009 — Little Rock Recruiting Office Shooting

Abdulhakim Mujahid Muhammad, an American, killed one member of the U.S. military and wounded another in a drive-by shooting of a recruiting office in Little Rock, Arkansas. While he is not officially charged as a terrorist, he told police he was "mad at the U.S. military because of what they had done to Muslims in the past," and his "intent was to kill as many people in the Army as he could."

2009 — Underwear Bombing Attempt

Umar Farouk Abdulmutallab, a Nigerian with connections to al-Qaeda, attempted to detonate a bomb sewn into his underwear while on a flight to Detroit.

2010 — Times Square Bombing Attempt

A car bomb — filled with propane tanks, fertilizer, gasoline, and firecrackers — failed to ignite in Times Square. Faisal Shahzad, a U.S. citizen born in Pakistan, was arrested and sentenced to life imprisonment for the plot. In 2009, he had traveled to Pakistan to learn bomb making from a militant extremist group. He was caught at JFK airport attempting to flee to Dubai.