Story highlights Peter Bergen: Many have tried, few have succeeded in terror bomb attacks

He says the last bombing carried out was in 2004, by a white supremacist

Bergen says authorities have been able to foil many bomb attacks

Scrutiny by law enforcement is greater and most bomb materials are restricted, he says

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.

Peter Bergen

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.

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.

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Almost overnight, the Oklahoma City attacks destroyed the scant credibility of the type of right-wing militia groups that McVeigh had associated with.

Photos: Deadly attack at Boston Marathon Photos: Deadly attack at Boston Marathon An injured man is loaded into an ambulance after two bombs went off near the finish line of the Boston Marathon on April 15, 2013. Three people were killed and at least 264 were injured. Hide Caption 1 of 55 Photos: Deadly attack at Boston Marathon A man comforts a victim on the sidewalk at the scene of the first explosion. Hide Caption 2 of 55 Photos: Deadly attack at Boston Marathon The second explosion goes off near the finish line. Hide Caption 3 of 55 Photos: Deadly attack at Boston Marathon The first explosion knocked down 78-year-old runner Bill Iffrig at the finish line. He got up a few minutes later and finished the race. Hide Caption 4 of 55 Photos: Deadly attack at Boston Marathon A runner embraces another woman on the marathon route near Kenmore Square. Hide Caption 5 of 55 Photos: Deadly attack at Boston Marathon Former New England Patriots offensive lineman Joe Andruzzi carries a woman from the scene. Hide Caption 6 of 55 Photos: Deadly attack at Boston Marathon A victim of the first explosion is helped on the sidewalk of Boylston Street. Hide Caption 7 of 55 Photos: Deadly attack at Boston Marathon An injured person is taken away from the scene in a wheelchair. Hide Caption 8 of 55 Photos: Deadly attack at Boston Marathon A woman kneels and prays at near the finish line. Hide Caption 9 of 55 Photos: Deadly attack at Boston Marathon People run down Exeter Street after the blasts. Hide Caption 10 of 55 Photos: Deadly attack at Boston Marathon 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. Hide Caption 11 of 55 Photos: Deadly attack at Boston Marathon Victims lie on the ground at the scene of the first explosion. Hide Caption 12 of 55 Photos: Deadly attack at Boston Marathon A couple runs from the scene pushing a stroller. Hide Caption 13 of 55 Photos: Deadly attack at Boston Marathon A runner reacts near Kenmore Square after the explosions. Hide Caption 14 of 55 Photos: Deadly attack at Boston Marathon A man's blood-stained feet hang outside an ambulance. Hide Caption 15 of 55 Photos: Deadly attack at Boston Marathon A bystander who was injured in the first explosion is wheeled across the finish line while receiving medical attention from rescue workers. Hide Caption 16 of 55 Photos: Deadly attack at Boston Marathon Carlos Arredondo was at the race handing out American flags to spectators. After the blasts, he helped emergency responders and is credited with helping a man survive serious leg wounds. Hide Caption 17 of 55 Photos: Deadly attack at Boston Marathon The second explosion goes off near the finish line. Hide Caption 18 of 55 Photos: Deadly attack at Boston Marathon Police look at blown-out windows. The bombs shook buildings, witnesses said, sending people to seek shelter under tables. Hide Caption 19 of 55 Photos: Deadly attack at Boston Marathon Unclaimed runners' bags fill an area near the marathon finish. Hide Caption 20 of 55 Photos: Deadly attack at Boston Marathon Women desperate to hear from loved ones are unable to get close to the site of the attack. Hide Caption 21 of 55 Photos: Deadly attack at Boston Marathon Police and runners stand near Kenmore Square after the attack. Hide Caption 22 of 55 Photos: Deadly attack at Boston Marathon Runners gather near Kenmore Square after the explosions. Hide Caption 23 of 55 Photos: Deadly attack at Boston Marathon An EMT worker is transferred to an ambulance outside a medical tent in Copley Square. Hide Caption 24 of 55 Photos: Deadly attack at Boston Marathon A SWAT team arrives on the scene. Hide Caption 25 of 55 Photos: Deadly attack at Boston Marathon A runner sits near Kenmore Square after the attack. Hide Caption 26 of 55 Photos: Deadly attack at Boston Marathon Runners who had not yet finished the race are stopped after the explosions. Hide Caption 27 of 55 Photos: Deadly attack at Boston Marathon Victims are helped at the scene of the first explosion. Hide Caption 28 of 55 Photos: Deadly attack at Boston Marathon A man in tears is helped at the scene on Boylston Street. Hide Caption 29 of 55 Photos: Deadly attack at Boston Marathon Passersby put pressure on a victim's leg to try to stop the bleeding. Hide Caption 30 of 55 Photos: Deadly attack at Boston Marathon Massachusetts State Police guard an area near Kenmore Square. Hide Caption 31 of 55 Photos: Deadly attack at Boston Marathon Bystanders embrace near the finish line. Hide Caption 32 of 55 Photos: Deadly attack at Boston Marathon Police officers gather on Newbury Street. Hide Caption 33 of 55 Photos: Deadly attack at Boston Marathon Women and children are escorted away from the scene. Hide Caption 34 of 55 Photos: Deadly attack at Boston Marathon Bomb squad officials check a possible suspicious device near the scene of the blasts. Hide Caption 35 of 55 Photos: Deadly attack at Boston Marathon A woman is comforted after the blasts. Hide Caption 36 of 55 Photos: Deadly attack at Boston Marathon Two injured women are taken away on stretchers. Hide Caption 37 of 55 Photos: Deadly attack at Boston Marathon An injured woman is loaded into an ambulance. Hide Caption 38 of 55 Photos: Deadly attack at Boston Marathon First responders load injured people into an ambulance. Hide Caption 39 of 55 Photos: Deadly attack at Boston Marathon An injured man is prepared to be moved from a stretcher to an ambulance. Hide Caption 40 of 55 Photos: Deadly attack at Boston Marathon An injured woman is placed on a stretcher. Hide Caption 41 of 55 Photos: Deadly attack at Boston Marathon A runner is comforted following the attack. Hide Caption 42 of 55 Photos: Deadly attack at Boston Marathon The Cambridge Police Department's bomb squad investigates unattended personal items left behind after the explosions. Hide Caption 43 of 55 Photos: Deadly attack at Boston Marathon Bystanders check their mobile devices for news of the explosions. Hide Caption 44 of 55 Photos: Deadly attack at Boston Marathon A member of the bomb squad investigates a suspicious item on the road near Kenmore Square. Hide Caption 45 of 55 Photos: Deadly attack at Boston Marathon A runner in a wheelchair is taken from a triage tent after the explosions went off. Hide Caption 46 of 55 Photos: Deadly attack at Boston Marathon People comfort each near the site of the blasts. Hide Caption 47 of 55 Photos: Deadly attack at Boston Marathon Racers and race officials stand by after the explosions. Hide Caption 48 of 55 Photos: Deadly attack at Boston Marathon Emergency personnel respond to the scene. Hide Caption 49 of 55 Photos: Deadly attack at Boston Marathon Police and emergency crews tend to victims. Hide Caption 50 of 55 Photos: Deadly attack at Boston Marathon An injured woman is carried away on a stretcher. Hide Caption 51 of 55 Photos: Deadly attack at Boston Marathon A man lies on the ground after the incident. Hide Caption 52 of 55 Photos: Deadly attack at Boston Marathon Officials watch as the first explosion goes off on Boylston Street. Hide Caption 53 of 55 Photos: Deadly attack at Boston Marathon Spectators leave the bleachers after the explosions. Hide Caption 54 of 55 Photos: Deadly attack at Boston Marathon Police inspect one of the blast sites. Hide Caption 55 of 55

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.