After an initial blast, a second explosion appears in the background near the finish line of the Boston Marathon.Getty Images
In a tragedy that no one could have anticipated, a pair of bombs near the finish line of the Boston Marathon exploded, killing three people and injuring over 100, according to reports available at the time of writing. The blasts were caused by compact explosive devices, but little else is known at this time, including who is behind the bombing and why.
"We still do not know who did this or why and people shouldn't jump to conclusions," said President Barack Obama at a White House press conference. "We will find out who did this and we will hold them accountable."
As unthinkable as the horror that visited the Boston Marathon was for the athletes and spectators attending the race, extremists have had a hand in scarring other major sporting events in the past.
A funeral is held for Jeyaraj Fernandopulle, the former Sri Lankan minister of highways, after he was killed in a suicide bombing at a marathon in Sri Lanka.Getty Images
As hard as it is to believe, five years ago, an attack that parallels the one that transpired today occurred half a world away.
In April 2008, a suicide bomber detonated an explosive at the start of a marathon in Sri Lanka. A dozen people, including a government minister, the national athletics coach and a former Olympian, died in the blast.
The bomber was suspected of being a member of the Tamil Tigers, a rebel group representing a predominantly Hindu minority that has been fighting for an independent state since 1983.
A funeral ceremony is held for the 11 Israeli Olympians murdered by terrorists during the 1972 Olympics in Munich.Getty Images
On Sept. 5, 1972, the most brazen and infamous act of terrorism at a sporting event took place when Palestinian terrorists murdered 11 Israeli athletes at the 1972 Olympics in Munich.
Gunmen belonging to the militant group Black September raided the Olympic Village where the athletes resided, and stormed two apartments in which Israeli Olympians were relaxing away from the field. The attack played out on television screens as images of masked men bearing guns were beamed to millions around the globe.
In the wake of the tragedy, competition was suspended, and a service was held for the fallen athletes.
Since the events of the 1972 Olympics, security has been considerably increased and has become a primary concern of event organizers. For the 2012 Olympics in London, security spending topped $1.6 billion.
A young woman visits a memorial to the victims of the bombing at Centennial Olympic Park in Atlanta.Getty Images
Nearly 24 years after the Munich massacre, tragedy would strike the Olympics again.
On July 27, 1996, a bomb detonated at Centennial Olympic Park during the 1996 Summer Olympics in Atlanta. The explosion killed two people and injured 111.
The terrorist in this case was an American, Eric Robert Rudolph, who was captured by authorities nearly seven years later. In that time, he committed three additional acts of terrorism, detonating three more bombs after the Games, two of which were at abortion clinics and one at a lesbian nightclub.
Rudolph claimed he bombed the games to bring down the Olympics and embarrass the U.S. government for legalizing abortion, according to USA Today. He received a sentence of life in prison without the possibility of parole.
Iraqi security personnel investigate the site at which where the head of Iraq's Olympic Committee, Ahmad el Kijiye, was kidnapped.Getty Images
Even before athletes take the field, terrorists can leave their mark on competitors.
On three different occasions in the same year, Iraq's Olympic team was targeted by violence. On May 17, 2006, members of the Iraqi national taekwondo team were kidnapped en route to a training event. That same month, the Iraqi national tennis coach and two tennis players were gunned down. Two months later, dozens of members of the Iraqi Olympic Committee were kidnapped.
Of the 38 members of the Iraqi national team that were kidnapped, only 13 were released alive and well. The others have never resurfaced.
A Chinese policeman stands guard at the National Stadium in Beijing.Corbis Images
Although terrorists may occasionally succeed in targeting major sporting events, their plots can be foiled in advance.
Chinese security forces managed to avoid a violent incident marring the 2008 Summer Olympics in Beijing when they allegedly foiled two terrorist plots.
Terrorists belonging to an ethnic separatist group attempted to hijack a jetliner in one incident. Communist party leaders also claimed to have uncovered a terrorist group that was planning to disrupt the Olympics.
Despite the potential severity of the alleged threats, a lack of details regarding the security operations led to some skepticism within China as to whether the danger was really there, according to TIME Magazine.
New Zealand cricket team captain Stephen Fleming gets emotional at a press conference following the suicide bombing.Getty Images
Cricket isn't a violent sport by any means. But there have been a number of high-profile incidents of terrorists attacking teams and disrupting events given the occasionally dangerous venues in which some matches are held.
In May 2002, a suicide bomber struck the hotel the New Zealand cricket team -- the Black Caps -- were staying during a tour of Pakistan. The bomb blast killed 14 people, but none of the players were injured.
The bombing forced the players to abandon their schedule and return home, at which time the team captain, Stephen Fleming (pictured here), gave an emotional press conference, stating that the incident would haunt him for the rest of his life.
The incident marked the second time in 15 years that the New Zealand witnessed this level of violence. In 1987, a car bomb outside a bus station that killed over 100 people in Colombo, Sri Lanka, forced the New Zealanders to suspend play.
A funeral is held for a policeman following the attack on the Sri Lankan cricket team.Corbis Images
In 2009, Sri Lanka's cricket team was en route to Gaddafi Stadium in Lahore, Pakistan, for its third day against the Pakistani national team. On their way to their destination, the bus was attacked by 12 gunmen.
Eight people died in the attack, including six police officers and two civilians. Five Sri Lankan players were also wounded.
The attack raised serious questions about the ability of Pakistan to host international sporting events given the uncertainty of security for players of any national team.
Debris following an explosion near Real Madrid's stadium is hauled away as spectators look on.Getty Images
With hours to go until a Champions League semi-final match between Real Madrid and arch rivals Barcelona FC, supporters of both teams eagerly anticipated the start of the contest. Outside the Bernabeu Stadium in Madrid, however, terrorists disrupted the festivities, detonating two car bombs, injuring 17 people.
The Basque terrorist group E.T.A. claimed responsibility for the attacks, calling a Basque newspaper to report their involvement mere minutes before the bombs went off.
Despite the bombing, the match proceeded as scheduled, with Union of European Football Associations (UEFA) conducting security sweeps before deciding game could go on.
Supporters of Togo soccer march against violence following an attack by a separatist group on the team.Getty Images
Togo is a tiny African nation that isn't very well known. But in 2006, their national soccer team made a name for itself with its impressive debut at the FIFA World Cup in Germany.
Four years later, the team made international headlines once again when it was the target of a terrorist attack. While the team was en route to Angola for the Africa Cup of Nations, a group of a dozen heavily armed separatist guerrillas ambushed the Togolese team bus. The gunmen killed three people -- the team's spokesperson, assistant coach and driver -- and wounded nine others on the bus.
The Front for the Liberation of the Enclave of Cabinda (FLEC) claimed responsibility for the attacks, stating that it had warned the Angolan government against holding the competition in the disputed oil-rich province, Cabinda.