WARNING: The above video may contain material considered disturbing and graphic.

Day one of the Boston Marathon bombing trial of Dzhokhar Tsarnaev proved to be a harrowing experience for the jurors privy to previously sealed evidence and eyewitness testimony, as well as for the witnesses who delivered that testimony.

One such witness was Shane O’Hara, manager of Marathon Sports on Boylston Street, who took the stand on Wednesday to share his account of what happened on April 15, 2013.

O’Hara’s statements were accompanied by the above surveillance video which shows the interior Marathon Sports before and after the first bomb exploded near the Marathon finish line.

“I was selling a pair of shoes to an old colleague when the bomb exploded,” remembered O’Hara.

Then the bomb combusted.

“I heard sound like firecrackers,” he added, and “saw smoke engulf the entire window.”

As seen above, one of the two-panes of the storefront window shattered. People came streaming into the store, some injured, and O’Hara sprang into action.

He fashioned a tourniquet for one person and tried entice others inside to safety.

O’Hara was one of six witnesses called on the first day of the high-profile trial.