WASHINGTON (AP) -- The Boston Marathon was still in progress on a bright Monday afternoon, but for Boston city police officers using the department's radio scanner channel, the day was winding down. At 2:51 p.m., an unidentified police dispatcher told several units that they could head out from the day's assignments.
"Dan, Sam, I'm all set," the dispatcher said. "You're all done for the day, Sam. Thank you so much, Dan."
The frequency went silent. Then, at 2:52 p.m. Monday, the police channel surged with the high-pitched shout of a police supervisor caught at the epicenter of urban devastation.
"Emergency! I need officers!" the supervisor called, his voice scrambled in static, then abruptly cut off by a second emergency call.
Two bombs had exploded near the Boston Marathon finish line. Shards of metal, ball bearings and nails sprayed out, slicing into scores of runners and spectators. In seconds, Boston's police and fire radio frequencies crackled with the frantic voices of officials confronted with the largest bombing attack on American soil since the 9/11 terror event.
Over the next several hours, the urgent and sometimes garbled appeals that flickered across Boston's emergency services radio bands gave full voice to the split-second and pressurized decisions that the city's police and fire uniformed and supervisory officials made under the extreme duress of a long, hard day. The scans show that it was not until 25 minutes after the bombing that an all-hands message let police know that a command post had been set up near the bomb site to anchor their response and investigation.
The voices recorded on the internet remain unidentified, but they testify to the real-time heroics that saved lives and created order out of chaos.
Moments after the two explosions, the Boston police dispatcher cut in to clear the radio channel so that only essential calls could be heard: "Units stay off the air! Units stay off the air! Just make your way over there. All units stay off the air and make your way over there! I only wanna hear from the 984, I only want to hear from that supervisor!"
At the same time, the Boston Fire Department's radio channel erupted in a cascade of static and electronic squeals -- paramedics and firefighters calling for reinforcements.
"671 Boylston," a firefighter bellowed. "We have multiple people down!"
The fire radio dispatcher echoed the man's warning, but the firefighter interrupted, impatient before he, too, tailed off. "We're gonna need as many ambulances as you can..."
A police supervisor stationed near the bombing epicenter took to the airwaves to summon medical help and all available police units to head directly to the chaotic scene on Boylston Street. But the supervisor also wanted the police channel kept clear for only urgent messages -- a tall order on a day when police found bleeding, stunned victims everywhere they turned.
"Delta 984," he shouted, identifying his call ID. "Here's what I need. I want reload cleared so I can get ambulances out, OK?"
His voice disappeared in a burst of static. Then: "I need lanes open here!"
"Everyone else stay off the air," the dispatcher warned.
"Get as many people as you can up here from the medical tent," the supervisor continued.
His voice dissolved in pleas from other officers. There were too many injured runners and spectators near the finish line.
"We're gonna need more ambulances here!"
"They're going, sir, they're going!" the dispatcher said.
"What's our crime scene?" asked an officer en route.
"755 Boylston," the supervisor replied.
"755 Boylston, crime scene, we have it," the dispatcher echoed.
On the fire frequency, an unidentified Boston officer warned incoming ambulances about the chaos waiting them. "We have hysteria in the street," he said. "All the streets are blocked with civilians. All companies use caution!"
A moment later, a fire supervisor crystalized the scene for those listening in. "We have reports of two explosions here. We have a multi-casualty incident here, at least a dozen serious injuries here."
"A mass casualty," echoed a woman's voice. It was the department's on-duty dispatcher.
For long minutes afterward -- in audio recorded on Broadcastify.com -- Boston's police and fire channels pulsed between officers pleading for ambulances to tend to gravely wounded victims and other officers warning about suspicious backpacks and bags strewn near the explosion scene. Suddenly, every unattended package seemed a potential bomb. The reports all later turned out unfounded, but at the moment, no package discarded on the ground could be ignored.