John Tlumacki/Globe Staff

Chaos ensued at the finish line of the Boston Marathon when two bombs exploded on Monday afternoon, injuring dozens and killing several.

David L. Ryan/Globe Staff

The day began as a beautiful, sun-filled, and joyful time as runners completed the 26.2-mile trek from Hopkinton to Boston.

David L. Ryan/Globe Staff

But the tenor of Marathon Day changed quickly when the first of two successive bombs exploded along Boylston Street just before 3 p.m.

David L. Ryan/Globe Staff

A short time later, the once-joyous finish line scene had become a crime scene under lockdown.

John Tlumacki/Globe Staff

The jolt of the first blast knocked this runner to the ground and prompted police to spring into action.

John Tlumacki/Globe Staff

Seconds later, another explosion rocked the street several blocks away from the finish line.

John Tlumacki/Globe Staff

To reach the injured, bystanders had to tear down a makeshift fence that had been built for the marathon.

John Tlumacki/Globe Staff

Police and marathon officials struggled to remove the barriers.

John Tlumacki/Globe Staff

The blast created a smoky haze that contributed to the confusion that reigned in the ensuing minutes.

John Tlumacki/Globe Staff

This man comforted one of the victims along the sidewalk.

John Tlumacki/Globe Staff

First responders sprang into action to help the injured people.

John Tlumacki/Globe Staff

A marathon official helped the runner who was thrown to the ground by the initial blast.

John Tlumacki/Globe Staff

The scene along the sidewalk near the bombing was gruesome.

David L. Ryan/Globe Staff

Wheelchairs that were on standby for runners who needed help at the finish line were put into use for those injured by the blasts.

John Tlumacki/Globe Staff

First responders wheeled away one of the injured on a stretcher.

John Tlumacki/Globe Staff

Another injured person was carried away by first responders.

Bill Greene/Globe Staff

Former Patriots offensive lineman Joe Andruzzi carried an injured woman away from the scene.

John Tlumacki/Globe Staff

This woman dropped to her kneels in reflection after the blasts.

Bill Greene/Globe Staff

The blasts were traumatic for many at the scene, and caused people to flee the area.

Yoon S. Byun/Globe Staff

Runners who had not yet finished the race were stopped on Commonwealth Avenue.

John Tlumacki/Globe Staff

The blasts blew out windows and left a trail of debris along Boylston Street.

David L. Ryan/Globe Staff

The blasts left an indelible image on the Boston Marathon, which will likely be forever changed by Monday's tragedy.