(CNN) -- Runner 19793 was reaching the finish line a little before 3 on Monday afternoon when the first bomb went off. Thunderous boom. Then a few seconds later, another.
Sounded like cannons on a battlefield.
Boston.com sports producer Steve Silva was covering the much-heralded Boston Marathon. He was shooting what should have been joyous finish-line scenes when in a few seconds, everything changed. His camera kept rolling amid screams of shock and horror.
"It was just immediately (evident) there were injuries, right in the middle of the spectator crowds. I saw dismemberment, I saw blood everywhere," Silva said.
"I saw someone lose their leg, people are crying, people are confused."
Rescuers rushed to the victims with stretchers and wheelchairs. Ambulances quickly lined up for blocks and blocks.
In between the screams of pain and panic were phone calls. "Mom, I'm safe."
They were words Boston Globe reporter Billy Baker heard many times as he kept passing people on the scene. He posted what he heard and saw on Twitter: "Finish line volunteers told to run. Describe fear 'like 9/11 or the tsunami.'"
He described a nervous calm energy as people either tried to figure out what was happening or had no idea where to go.
Then his tweets got considerably more grim:
"Now getting gruesome first-hand accounts of hair on fire, severed limbs, battlefield scene in front of Charlesmark Hotel."
Confusion. Bewilderment. Rumors everywhere.
"It's not safe to be here," said a Boston police officer evacuating Commonwealth Avenue, Baker reported.
Jim Bardin works in an office building between the locations of the two blasts.
"I heard the first blast and it shook the building a bit, and went to see what was going on and the second one went off a couple of seconds after," Bardin said.
What he saw was mayhem below.
"People were pretty panicked down there -- the crowd was trying to get away as fast as possible. From up above, it looked like mayhem.
"I thought twice about being near the window for a while in case there were more. There wasn't much I could do out there. They weren't going to let anyone closer -- our front door goes right out in between them."
Will Ritter was about a block away, near Copley Square. He was trying to arrange a press conference for a runner who had just finished the race.
He said the blasts felt and sounded like the concussion bursts at the end of a Fourth of July fireworks show. Then he saw the white smoke billowing. Then emergency vehicles -- and pandemonium.
"Let's go, Let's go," shouted rescue workers.
"Oh my god, Oh my god," shouted people who were witness to the carnage.
Mark Gordon had just moved to his high-rise apartment on Boylston Street a month and a half ago. He had a perfect view of the marathon from his balcony and throughout the day, he had looked out and snapped photos.
It was a glorious day in Boston, the city he'd lived in for 12 years.
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He was doing household chores when the first bomb went off. It sounded like a thousand steel garage doors slamming shut all at once. He knew immediately it wasn't a conventional Boston city sound.
An expletive automatically rolled off his tongue. He ran to a window to see the second blast through a gap in the buildings across from him.
His mind went blank for a second or two. Then he grabbed his camera again and began clicking.
"I'll never look out my window the same way again even though it's been six short weeks," Gordon said.
Shortly after that, he was evacuated from his building. He's staying at a friend's house Monday night.
"I'm really, really mad about what someone's done to my city. I'm sick to my stomach. I'm also very proud," he said about the police and first responders.
Erin Farley, a New Jersey native and Emerson College student, was in third grade when the attacks of September 11, 2001, occurred. She wasn't old enough to understand then the gravity of what was happening.
Monday, in Boston, it was different.
"I have a chill that won't leave my body right now," she said.
Brittany Smith, a physical therapy student at Northwestern University in Boston, was volunteering at Medical Tent B, two blocks away from the finish line. She was treating runners for common ailments like muscle cramps when she heard the first loud bang.
Everyone just looked at each other wondering whether it was a celebratory cannon.
"Everyone's like, 'What's going on?' You could just sense something was wrong, that things were definitely not right," Smith said.
"We didn't rush to the scene, I was trying to (help) a marathoner locate her family members ... and I'm freaking out. It was really hard to focus on helping out the marathon runner. I was just panicking," she said.
She and other volunteers were desperate for information. They turned on the news and saw the footage from helicopters whirring above.
The sidewalks had turned crimson.
Smith was lucky in that a fellow volunteer had a car and dropped her off at her grandmother's house a couple of miles away.
They met her at the door with enormous hugs.