MINNEAPOLIS, Minnesota (CNN) -- The first caller to reach 911 reported -- in a calm voice that gave no hint to the calamity that awaited -- "a bridge down over the river."
Hours after Wednesday's collapse, rescuers searched for victims in crumpled cars.
Four minutes later, just before 7 p.m. Wednesday, Sgt. Ed Nelson, 55, and two fellow police officers were among the first to arrive at the south side of the Interstate 35W bridge that rose 60 feet above the Mississippi River.
"I don't think anybody quite expected the scene we encountered," he said at a news conference featuring first responders. "It was utter chaos at that time -- bridge smoking, dust in the air. ... We knew it was a bad, very bad situation."
The trio wasted no time. They split up, leaving one officer on the road to direct arriving first responders while Nelson and a fellow officer ditched their gunbelts and their radios and clambered aboard a span of the bridge that had tumbled to the river, carrying a still-undetermined number of vehicles and people with it.
With some people still in their cars, some on the fallen bridge span and some in the water, the men sprang into action.
"You forget about everything else," said Nelson, a former soldier who has been a police officer for 25 years, 20 of them with the Minneapolis Police Department. "You understand that you have a job to do and it's time to get down to business and do that job."
The first vehicle they came across was submerged and crushed, he said.
"We asked a man if he saw anybody get out of that vehicle, and he said, 'That was me,' " Nelson said.
As the officers crawled on a beam, the span "seemed to shift," and they could hear rivets popping amid victims' cries for help, Nelson said.
"We were forced to make a decision: Do we stand back and watch, or do we take action?" he said.
"It's not a place that I would care to go again, but you take a calculated risk and you do your job."
Within a few minutes, fire and rescue officers were helping the men. Three victims with possible back injuries were placed on boards and carried to safety.
"Basically, we just told them to be calm," Nelson said. "A couple of girls were somewhat hysterical. They couldn't understand how they got to that point. I just explained to them that they fell. They seemed somewhat amazed by that fact."
Nelson said he told them, "It's not the fall that hurts you -- it's the sudden stop at the end. They found that amusing, and it calmed them down."
Only after the police officers had made sure everyone was off the span did the impact of the work hit home.
"When you're doing it, emotion doesn't enter into it," Nelson said. "You understand fully that these people are depending on you, and you have a job to do. And that's what you do."
But, he added, "12 hours later, you look at the bridge and you go: 'I haven't a clue why I was under that.' "
Much of the rescue work was achieved only after overcoming logistical barriers. About 10 minutes after the collapse, John Hick of the Hennepin County Medical Center was one of the first doctors to arrive at the south side of the wreckage site.
But, unable to reach victims from there, he crossed to the north side of the site, where he helped other first responders carry people from the water.
The tough terrain on the riverbank made getting ambulances to the victims tough, so the first responders commandeered pickup trucks to ferry 55 victims from the water's edge, Hick said.
As Nelson and Hick were toiling, firefighter and diver Shanna Hanson arrived on the north side, where, with just a rope tied around her waist, she dove repeatedly into the water, searching vehicles for possible survivors.
Though such work -- swift-water rescue -- can be extremely dangerous, "you don't think about it much until afterward," she said, noting that she focused Wednesday on staying away from live wires and "widow makers" -- overhangs that could collapse at any time, killing anyone below.
But the work went smoothly, aided by years of having participated in disaster drills, said the diver, who has been a member of the Minneapolis Fire Department since 1991. "It was not frantic; it was pretty organized."
One of the most difficult parts of the entire operation had nothing to do with rescue work. Instead, it involved breaking the time-honored tradition shared by many firefighters of shunning the public spotlight.
"The media part of it is very difficult for us," Hanson told reporters, before ending the news conference abruptly. E-mail to a friend