Editor's Note: To follow ongoing developments in the investigation of the Las Vegas shooting, including changes in the sheriff's timeline of the crime, go to CBSNews.com.

The gunman at the Mandalay Bay hotel in Las Vegas who shot into a crowd of thousands did it from a room on the 32nd floor. He may have killed many more if not for a security guard who arrived on the floor within 12 minutes of the onset of the attack. Soon, a small group of Las Vegas police arrived. They organized themselves into an ad-hoc SWAT team and began storming room 135.

Members of the law enforcement team who were the first to enter Stephen Paddock's hotel room after he opened fire on a crowd in Las Vegas. CBS News

Tonight, for the first time you'll hear details of the assault from the officers who carried it out.

One week ago, just after 10 o'clock at night, 22,000 people became targets at a country music concert. Las Vegas detective Casey Clarkson was among them.

Casey Clarkson: The fire just starts hitting us. And you just hear--

[Body cam: They are shooting right at us, guys, everyone stay down, stay down.]

Casey Clarkson: I just remember, like, it was, like-- like, white spark, like, powder almost, like, hitting the concrete, hittin' the van. I mean, I'm just watching these rounds hit all, like, right next to us. And I'm like, "How is he so accurate?"

Detective Casey Clarkson CBS News

Bill Whitaker: Bullets just raining down on--

Casey Clarkson: --it's just raining down the whole time. And I just remember, I'm like, I'm just looking at my gun. I'm just like, "I got a pistol in my hand and this guy's shooting at me with an automatic rifle."

Bill Whitaker: You got grazed. But you-- you have shrapnel.

Casey Clarkson: Yeah, I got a piece of shrapnel in my neck. And I grabbed it for a second. And it was already just pouring blood, dripping off my hand and I was like, oh, my goodness.

Casey Clarkson: And I wanted to try to do more. But my partner was worried because I was getting light-headed at one point.

Casey Clarkson: And I said, "You know what, I'm just gonna stay. And I'm gonna help as many people as I can."

Bill Whitaker: Doing this while your neck is bleeding.

Casey Clarkson: Yes, sir.

Las Vegas police released department body cam footage of the chaos on the ground as Stephen Paddock fired from above.

Sergeant Joshua Bitsko CBS News

Joshua Bitsko: And-- so I hear-- probably Casey's radio traffic-- of, you know, he-- "We're takin' fire. Don't go down the boulevard."

[Police radio: We're trying to see where the shots are coming from if anyone can advise that they're coming from the Mandalay.]

Joshua Bitsko: I just yell to these guys, "Let's go. There's an active shooter."

Sergeant Joshua Bitsko and Officer Dave Newton of the K9-unit had been training dogs when they heard the call.

Joshua Bitsko: So we jump in our cars and hop on the freeway and we were there probably within five minutes.

Detective Matthew Donaldson was doing paperwork at headquarters. He sped nine miles to the Mandalay Bay. In the chaos, he had to run the last few blocks. His cowboy boots rubbed his feet raw.

Matthew Donaldson: I took my boots off. I just threw 'em in the casino. That was slowing me down. I was faster barefoot and I was gonna be more effective barefoot.

Officer David Newton CBS News

Bill Whitaker: How did you end up being first responders? Are you assigned to be first responders? Or you just got up and ran in-- into the danger zone and got there first?

Dave Newton: Correct. We heard it over the radio. We heard, you know, active shooter. Multiple victims.

Joshua Bitsko: We're told that security is taking fire from a suspect on the 29th floor. And that we had other officers that were identifying the suspect was in a room on the 32nd floor. So we're thinking multiple shooters at this point.

The three officers zeroed in on the 32nd floor just after a hotel-security guard named Jesus Campos encountered heavy fire--200 rounds shot into the hallway from behind Stephen Paddocks's door.

Officers Bitsko and Newton ran to the stairs. Detective Donaldson and a SWAT officer, Levi Hancock, did too. This ad-hoc group of officers didn't know what they would face, but they soon discovered Paddock had barricaded the stairwell door.

Joshua Bitsko: He had screwed shut the door-- with a piece of metal and some screws.

Detective Matthew Donaldson CBS News

Matthew Donaldson: In the stairwell.

Joshua Bitsko: In the stairwell going out to the hallway right by his door.

Dave Newton: 'Cause he knew we'd be coming out that door to gain entry into his door. So he tried to barricade it as best he could. But thankfully Levi had-- a pry bar and was able to easily pop that door.

The K9 cops, the detective, and the SWAT officer were now a team.

Bill Whitaker: So essentially you became the SWAT team.

Dave Newton: Uh-huh (affirm)

Joshua Bitsko: Yes.

Matthew Donaldson: Pretty much.

[Police radio: You need to be careful of booby traps...are you coming up the stairwell of coming up the elevator.]

Bill Whitaker: I heard on the radio calls that they were telling you to watch out there could be booby-traps.

Dave Newton: There's a room service cart with wires going on it underneath the-- door. There was something black on top of the cart. So initially I'm, you know, I'm thinking, "This is a booby-trap. It's-- it's gonna explode."

Bill Whitaker: 'Cause it looked suspicious?

Dave Newton: Very suspicious. It turned out to be cameras on the food tray.

Bill Whitaker: What'd you see?

Dave Newton: I could see the suspect's door was just riddled with bullet holes coming out. it looked like Swiss cheese.

Bill Whitaker: He had the advantage.

Dave Newton: Yeah, he-- 'cause he knew we were coming and we were gonna have to come through. We didn't know where he was gonna be in that room.

Joshua Bitsko: It's like a deadly game of hide and seek because when you're the one hiding you always know a person's looking for you it's-- before they see you. I remember thinkin', "Man, I wish I had my dog with me," because, you know, it's nice to have him lead a team.

Levi Hancock, the SWAT team member, was armed with explosive charges to blow through doors. Around 11 p.m. the team began to execute a plan. They had heard no gunfire since reaching the hallway, and had no idea what or who was behind the door. David Newton had a hand-held ballistic shield.

Dave Newton: Now I'm standing out in front of this bullet-ridden door with nothing except for a shield that's, you know, I'm hoping would help a little bit. And-- that was the point I said-- I just start praying that nothing goes off of phone wise or radio or anything else 'cause we're trying to be as quiet as we can 'cause we didn't want him to know we were out there and start spraying at us. And I'm watchin' Levi put the charge on 'em. I'm like, "Hurry, hurry, hurry, but be quiet." And-- so then we got it hung and then we retreated back into the stairwell, blow the door.

Police radio: We need to pop this and see if we can get any type of response from this guy – to see if he is in here or if he's actually moved on somewhere else.

60 Minutes correspondent Bill Whitaker CBS News

Dispatcher: All units on the 32nd floor SWAT has explosive breach. Everyone in the hallway needs to move back. All units need to move back.

Officer: Breach. Breach. Breach.

Bill Whitaker: So you blow the door open. And what do you hear? Were there fire alarms going off?

Dave Newton: Yes, the fire alarms were going off--

Bill Whitaker: The explosion set off the fire alarm.

Joshua Bitsko: Yes.

Bill Whitaker: So you enter the room. What do you see?

Joshua Bitsko: An armory. Just--

Bill Whitaker: Armory?

Dave Newton --so many guns. So many magazines. Stacks and stacks of magazines everywhere. Just in suitcases all neatly stacked against pillars, around the room, all stacked up, rifles placed all throughout. All kinds of monitors and electrical equipment he had in there. It just looked like almost a gun store.

Joshua: Shell casings all over the floor. I could smell the -- gun powder that -- that had went off in the room. We were trippin' over guns. Trippin' over long guns inside. There was so many.

Bill Whitaker: That many.

Dave Newton: Yes.

Matthew Donaldson: My initial scan, coming in the room with my rifle is just seein' I'm seeing one male down, bleeding from the face. He was not a threat. Kept going, kept going, kept going.

Bill Whitaker: Said one male down. That was the shooter?

Matthew Donaldson: Yes.

Bill Whitaker: Stephen Paddock?

Bill Whitaker: What were his wounds?

Dave Newton: I didn't see any apparent wounds to his head. But I did see a lotta blood that had come outta his mouth.

Joshua Bitsko: There was -- a bloody revolver I think -- nearby. Nearby him that was on the ground consistent with him shootin' himself.

Bill Whitaker: What else did you see?

Dave Newton: I saw a few phones-- laptops, a couple laptops he had in there. A lot of drills-- drill bits, all kinds of tools

Bill Whitaker: This was eerie.

Dave Newton: Very eerie. Yeah. The dust from the explosive breach. And then you have the flashing lights. And that looked straight-- like, out of a movie, you know? I did notice a note on the nightstand near his shooting platform. I could see on it he had written the distance, the elevation he was on, the drop of what his bullet was gonna be for the crowd. So he had had that written down and figured out so he would know where to shoot to hit his targets from there.

Bill Whitaker: What were the numbers? I am just trying to understand, were they calculations?

Dave Newton: Yeah, he had written we must have done the calculations online or something to figure out what his altitude was going to be and how high up he was-- how far out the crowd was going to be and what at that distance and what the drop of his bullet was going to be. He hadn't written out the calculations all he had was written out he final numbers that were on the sheet.

Bill Whitaker: Wow.

Bill Whitaker: Did you go to the window? Did you look out?

Joshua Bitsko: No. Because I know that S.W.A.T. had deployed snipers also. So I didn't wanna put my silhouette in front of a window-- because communication was still horrendous at that point

Bill Whitaker: Detective Donaldson, you came in and went through to the other room.

Matthew Donaldson: Yeah.

Bill Whitaker: What did you see?

Matthew Donaldson: It was still very much in my brain there's 50 other dudes in here somewhere. You know, we were still clearing that room, the curtains, moving the curtains. I wanted to make sure somebody wasn't hiding between the windows and the curtains. It's a very small bedroom. Cleared the bathroom, cleared the shower. Came back out. They had to breach the other door on the other side. And then -- once that scene was static it was -- it was essentially a crime scene. It was, like, stop what you're doing. Get out.

Bill Whitaker: It seems that he chose the tactically perfect and horrible spot for him to be able to rain down death on the people below him.

Dave Newton: Yeah, he did his homework.

Joshua Bitsko: Days of planning. Days of planning. He had tool boxes in power tools to run wires for his-- surveillance systems. For everything that he had, it took him days to finish.

Bill Whitaker: The sheriff was saying the other day that it -- it almost appeared as though he thought he would be able to get out of this, that he had an escape plan. Did you see any evidence of that?

Dave Newton: From what I saw, it -- I thought he was gonna -- he -- his plan might have been to shoot it out with us. Because there was a rifle on a bipod near the door and just the amount of ammunition and weapons he had. He could of held us off for hours.

Joshua Bitsko: But at least if he was shooting at us, he wasn't shooting at Casey and all the victims down there at the concert.

Las Vegas Sheriff Joseph Lombardo CBS News

Joseph Lombardo: Before, we were trained to form a perimeter and hope for the best. Now we're trained to gather up and go get it.

Las Vegas Sheriff Joseph Lombardo traveled to Mumbai, India, after the 2008 terrorist attacks that targeted numerous sites including hotels across the city. Hundreds were killed and wounded. Lombardo learned a key lesson: officers should immediately react to stop a shooter at any cost. His department has made that a pillar of its training ever since.

Bill Whitaker: Was this the scenario, the horror you had trained for?

Joseph Lombardo: Absolutely, cause that's how we train them And through all that experience and knowledge and training that we put together -- a team was immediately formed, acting on their own without supervisory direction -- Get a group of officers together as fast as you can. And immediately address the threat to cease the action.

No one knows when Paddock committed suicide, but the shooting stopped shortly after security guard Campos and the first of Sheriff Lombardo's officers arrived on the 32nd floor.

Bill Whitaker: Your guys got up there in, like, 12 minutes.

Joseph Lombardo: Twelve minutes. Twelve minutes. And--

Bill Whitaker: Up on the thirty-second floor--

Joseph Lombardo: You know, and during a critical incident, 12 minutes is a long time, you know? Could you imagine being in a fistfight for 12 minutes? It's a very long time. But when you step back and you evaluate it after the fact, it's a s-- very short period of time for th-- to get the intelligence, figure out what the hell is going on-- put a team together. Go up 32 floors and evaluate the situation. I think they prevented a thousand deaths. And I think it's important for the American public to understand that.

Produced by Marc Lieberman, Ali Rawaf, Michael Rey and Oriana Zill de Granados. LaCrai Mitchell, associate producer.

Credit: Bild Exclusive/Polaris