The Boston Marathon is one of the most challenging tests for any marathon runner. The competition has been considered the pinnacle of the sport for several years. This year, at the 2013 Boston Marathon, shortly after Lelisa Desisa Benti won the men's division of the competition and Rita Jeptoo won the women's division, the course was completely side-swiped by a series of bombs that exploded near the finish line of the event.

The series of explosions were filmed by innocent bystanders and journalists wielding camera phones among other recording devices.

The Boston Marathon explosion can be seen most clearly from a camera that the Boston Globe was using to film runners as they crossed the finish line. The footage shows that as dozens of runners spill across the finish line, a gigantic explosion takes place at the north side of Boylston Street. As the explosion hits, dozens of screams and cries from innocent bystanders can be heard.

A man and women finishing the marathon continue to jog toward the finish line, though they both look back to determine how much damage has been created by the explosion. The Boston Globe cameraman runs toward the explosion and continues to film as one runner lays on the ground holding his leg. He films the plume of smoke as it billows out from behind a row of flags depicting all the nations represented at the marathon. As you may imagine, pandemonium completely takes over the race. People begin jumping gates and blowing whistles trying to to help. "We've had an attack," whispers the cameraman.

The explosion reportedly caused two deaths and left more than two dozen people injured. There were two explosions that occurred on the course of the Boston Marathon itself, and later in the evening, just before 4 p.m., another explosion reportedly occurred at the JFK Library. "Eyewitnesses said the explosions were about 15 seconds apart. A few runners stopped and turned around after they heard the blasts. Some runners were seen crying and some said they saw limbs on the ground," reports the New York Times.

Social media sites such as Twitter and YouTube have been filled with photos, video and other messages about the details of the attack. In many of the videos, people can be seen literally running toward the explosion to help individuals hurt or impacted by the blasts. Please be warned: The images and footage surrounding this horrific event are startling, graphic and may not be suitable for everyone. Please view all media surrounding the 2013 Boston Marathon explosions at your own risk:

Here are photos of the 2013 Boston Marathon explosion circulating Twitter: