Freelance photographer Bill Hoenk was on hand to document the chaotic aftermath immediately following the second explosion near the finish line of the Boston Marathon. His photographs, which may be the only images recording the scene of the second blast, show a Boston police officer carrying a wounded child. One of Hoenk's photographs is the cover image of TIME's May 6, 2013 digital edition.

One of the most important eyewitnesses to the Boston Marathon bombing never took the stand and never said a word to the jury. But what Bill Hoenk saw that day in April 2013 was key to the prosecution's case against Dzhokhar Tsarnaev, CBS News' Don Dahler reports from Boston.

Nantucket photographer Bill Hoenk was just forty feet away from the finish line when the bombs went off at the Boston Marathon. In a matter of seconds, he went from capturing the jubilation of faces of runners to documenting the worst terror attack on US soil since 9/11. In the days and weeks that followed, Hoenk’s photos not only appeared on the cover of Time, but they also served as crucial pieces of evidence in convicting Dzhokhar Tsarnaev. Now, four years later, Hoenk — who recently participated in the filming of Mark Wahlberg’s Patriots Day — reflects on the horrors of that fateful day and his personal journey to return to the race.

Although it’s not lost on me that the photos I took are of real people, very sensitive in nature, and have caused quite a bit of grief and sadness to some, it also helps knowing that, over time, those same photos have come to help as well, so when I noticed a random article online that Boston Casting was looking for extras to be in Mark Wahlberg’s new movie about the marathon bombings, “Patriots Day,” I thought, maybe I can help in some way again?