The audio was released by Boston EMS on Thursday and details the terrifying moments following the twin explosions that left three people dead and more than 260 injured.
"What was that on Boylston Street?" was the first radio transmission after the blasts, followed by: "Something just exploded at the finish line."
First responders spent the next moments getting a handle on the situation. Among the transmissions in the next six minutes:
"We are confirming two devices just went off at Boylston and Exeter, all units extreme caution."
"We just had an explosive device go off by the Copley library."
"All units hold your positions until we get a confirmation on the injuries."
"Return to your vehicles, please, everybody."
Officials at the scene can be heard coordinating with dispatchers to get emergency crews to the proper areas.
"We don't have a count yet on casualties, but we do have casualties coming in…there is potential here for a mass casualty event."
"Exeter and Boylston. I've got three people down. Heavy trauma."
"I have multiple casualties on Boylston."
The Boston Globe reports that first responders used the audio as part of their yearly review of how crews handled the Boston Marathon.
Officials told the Globe that about 100 EMS personnel worked the scene from Brighton to Copley. Immediately after the bombings, in addition to Boston EMS, 60 private ambulances responded to the incident.