On April 15, 2013, two bombs exploded in the crowded street near the finish line of the Boston Marathon, killing three people and injuring more than 140 others. It was the latest in a series of terrorist attacks on sporting events going back to the 1970s. See all photography related to the Boston bombings.
Editor's note: Peter Bergen is CNN's national security analyst, the author of "Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad" and a director at the New America Foundation. Jennifer Rowland is a program associate at the New America Foundation.
Washington (CNN) -- It's too early to tell who is responsible for Monday's bombings in Boston. Yet after an incident like this, everyone is looking to find out who did it and why.
One possible guide is recent history: In the years since the 9/11 attacks, dozens of extremists have plotted to use explosives to further their causes in the United States.
Of the 380 individuals indicted for acts of political violence or for conspiring to carry out such attacks in the U.S. since 9/11, 77 were able to obtain explosives or the components necessary to build a bomb, according to a count by the New America Foundation.
Of those, 48 were right-wing extremists, 23 were militants inspired by al Qaeda's ideology, five have been described as anarchists and one was an environmentalist terrorist.
But in the years since 9/11, actual terrorist bombings in the U.S., like the ones at the Boston Marathon, have been exceedingly rare.
The only bombing attack carried out by an extremist in the United States during the past 12 years was in 2004 when Dennis Mahon, a white supremacist, sent a homemade bomb to Don Logan, the African-American city diversity director of Scottsdale, Arizona, who was maimed when the package exploded in his arms.
By contrast, in the decade before 9/11, the United States saw a number of terrorist bombings -- such as the 1993 truck bomb that killed six at the World Trade Center, carried out by a group of men inspired by al Qaeda's ideology; the Oklahoma City bombing two years later, which killed 168, masterminded by Timothy McVeigh, who was motivated by right-wing extremist ideas; and the 1996 Olympic Park bombing in Atlanta, carried out by anti-abortion extremist Eric Rudolph, which killed one person.
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There are several reasons for the decline in the number of successful bombing attacks by violent extremists in the years since the Oklahoma City bombing.
An injured man is loaded into an ambulance after two bombs went off near the finish line of the fabled Boston Marathon on Monday, April 15. For the latest details, read CNN's developing news story.
Photos: Deadly attack at Boston Marathon
Almost overnight, the Oklahoma City attacks destroyed the scant credibility of the type of right-wing militia groups that McVeigh had associated with.
The feds also began to pay considerable attention to anyone purchasing large amounts of fertilizer of the kind that was used to construct the Oklahoma City truck bomb.
After 9/11 there was a rapid increase in the number of Joint Terrorism Task Forces around the country, which are made up of multiple law enforcement agencies working together to ferret out suspected terrorist activity.
And following the 9/11 attacks, far more businesses started reporting to law enforcement suspicious purchases of any kind of material that could be used for bomb-making.
As a result, since 9/11 bomb plots that have simply fizzled out have overwhelmingly been the rule.
Around the eighth anniversary of 9/11, for instance, an al Qaeda recruit, Najibullah Zazi, plotted to detonate a number of bombs on the Manhattan subway system.
During the summer of 2009 Zazi, a resident of Denver, obtained a large amount of hydrogen peroxide from a beauty store in Aurora, Colorado. Hydrogen peroxide-based bombs are something of a signature of al Qaeda plots in recent years. The chemical can be used to make the same type of explosives that were detonated during the 2005 London transportation system bombings, in which 52 commuters were killed, a plot that was directed by al Qaeda. Zazi was arrested before he could carry out his plan.
Al Qaeda terrorists have increasingly turned to using hydrogen peroxide-based bombs because it has become very difficult for militants based in the United States and other Western countries to buy conventional explosives.
The plot to bring down as many as seven British, American and Canadian planes leaving London's Heathrow Airport in the summer of 2006 is emblematic of this. A group of al Qaeda-linked British terrorists conspired to manufacture hydrogen peroxide-based bombs, which they planned to smuggle onto flights in soft drink bottles. The plot failed, but a continuing legacy is the fact that passengers cannot bring substantial amounts of liquids onto flights.
Less than a year after the Zazi plot was foiled, on May 1, 2010, Pakistani Taliban recruit Faisal Shahzad tried and failed to detonate a bomb consisting of propane, gasoline and fireworks in an SUV parked in Times Square.
While terrorists working for al Qaeda or allied groups have conspired to launch a number of bombing attacks in the United States in the years since 9/11, right-wing extremists have also been quite active.
In January 2010, Kevin Harpham, a white supremacist, placed a backpack filled with explosives along the route of a Martin Luther King Jr. parade in Spokane, Washington. Fortunately, city workers spotted the suspicious package and police were able to defuse the bomb before the parade began.
Three years earlier the feds arrested a group of men in Alabama calling themselves "The Free Militia" who had manufactured 130 homemade hand grenades.
By contrast to the dozens of right-wing extremists and al Qaeda-inspired militants who have obtained explosives or bomb components, since 9/11 there has been only one case of an environmental militant doing so.
On September 1, 2010, James J. Lee entered the headquarters of the Discovery Channel in Silver Spring, Maryland, with explosives strapped to his chest and guns in his hands. Before he was killed by police, Lee took several people hostage and demanded that more attention be paid to environmental issues.
While the perpetrator or perpetrators of the Boston bombing are as yet unknown, an analysis of the 77 people based in the United States who have assembled bomb-making materials or tried to carry out a bombing for political purposes since 9/11 shows they have overwhelmingly been motivated either by al Qaeda-like ideas or right-wing extremist ideology.
A key to determining who is responsible for the Boston bombings will be the evidence obtained from the two bombs.
So far, what is known about these explosive devices doesn't point to any particular group or type of individual.
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The 1972 Munich Olympics is remembered for tragedy, rather than sporting excellence, after 11 Israeli athletes were killed by the Palestinian terrorist group known as Black September.
The Olympics were targeted again in 1996 when a bomb at Centennial Park killed one and injured hundreds of others during a free concert in the U.S. city of Atlanta.
Between May and July 2006, Iraqi sportspeople were the target of a number of attacks. The 15 members of the Taekwondo team were kidnapped and never seen again, while a national tennis coach and two players were killed following an attack on a sports conference in Baghdad.
In April 2008, a suspected Tamil Tiger suicide bomber detonated a device at the start of a marathon celebrating Sri Lanka's New Year. Highways minister Jeyaraj Fernandopulle, former Olympic marathon runner KA Karunaratne and the national athletics coach, Lakshman de Alwis, were among the dozen people killed.
New Zealand cricket captain Stephen Fleming was forced to hold back the tears in May 2002, after his team ended its tour of Pakistan when a suicide bomber attacked outside the team's hotel in Karachi. Fourteen people were killed, including 11 French Navy experts, two Pakistanis and the Pakistan team's physiotherapist.
Sri Lanka's national cricket team was evacuated by helicopter during its tour to Pakistan in 2009 after seven people were killed when the team bus was attacked in Lahore.
Three people were killed and nine were injured when the Togo football team bus was ambushed in Angola before the 2010 Africa Cup of Nations. Rebels used machine guns on the vehicle as it traveled from the Democratic Republic of Congo into Angola's oil-rich region of Cabinda.
A man comforts a victim on the sidewalk at the scene of the first of two apparent bombings.
A second explosion goes off near the finish line.
A runner embraces another woman on the marathon route near Kenmore Square.
Former New England Patriots offensive lineman Joe Andruzzi carries a woman from the scene.
A victim of the first explosion is helped on the sidewalk of Boylston Street.
An injured person is taken away from the scene in a wheelchair.
A woman kneels and prays at near the finish line.
People run down Exeter Street after the blasts.
The explosions occurred around 2:45 p.m., about an hour after the first of the race's nearly 27,000 runners had crossed the finish line.
Victims lie on the ground at the scene of the first explosion.
A couple runs from the scene pushing a stroller.
A runner reacts near Kenmore Square after the explosions.
A man's blood-stained feet hang outside an ambulance.
A bystander who was injured in the first explosion is wheeled across the finish line while receiving medical attention from rescue workers.
Carlos Arredondo, who was near the finish line when the bombs detonated, leaves the scene. He was at the race handing out American flags to spectators. After the blasts, he helped emergency responders and is credited with helping a man with serious leg wounds survive.
The second explosion goes off near the finish line.
Boston police look at blown-out windows. The bombs shook buildings, sending people to seek shelter under tables, witnesses said.
Unclaimed runners' bags fill an area near the marathon finish.
Women desperate to hear from loved ones are unable to get close to the site of the attack.
Police and runners stand near Kenmore Square after the attack.
Runners gather near Kenmore Square after the explosions.
An EMT worker is transferred to an ambulance outside a medical tent in Copley Square.
A SWAT team arrives on the scene.
A runner sits near Kenmore Square after the attack.
Runners who had not yet finished the race are stopped after the explosions.
Victims are helped at the scene of the first explosion.
A man in tears is helped at the scene on Boylston Street in Boston.
Passersby put pressure on a victim's leg to try to stop the bleeding at the scene of the first explosion.
Massachusetts State Police guard an area near Kenmore Square.
Bystanders embrace near the finish line.
Police officers gather on Newbury Street.
Women and children are evacuated from the scene.
Bomb squad officials check a possible suspicious device near the scene of the blasts.
A woman is comforted after the blasts.
Two injured women are taken away on stretchers.
An injured woman is loaded into an ambulance.
First responders load injured people into an ambulance.
An injured man is prepared to be moved from a stretcher to an ambulance.
An injured woman is placed on a stretcher.
A runner is comforted following the attack.
The Cambridge Police Department's bomb squad investigates unattended personal items left behind after the explosions.
Bystanders check their mobile devices for news of the explosions.
A member of the bomb squad investigates a suspicious item on the road near Kenmore Square.
A runner in a wheelchair is taken from a triage tent after the explosions went off.
People comfort each near the site of the blasts.
Racers and race officials stand by after multiple explosions near the finish line.
Emergency personnel respond to the scene.
Police and emergency crews tend to victims.
An injured woman is carried away on a stretcher.
A man lays on the ground after the incident.
Officials watch as the first explosion goes off on Boylston Street in Boston.
Spectators leave the bleachers after the explosions.
Police inspect one of the blast sites.
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