The following is a script from "Active Shooter" which aired on Nov. 22, 2015. Anderson Cooper is the correspondent. Ira Rosen and Habiba Nosheen, producers.
The coordinated strikes in Paris, carried out by terrorists at multiple locations, as well as the attack this past Friday in Mali, are the latest examples of what American law enforcement calls "active shooter cases."
These are situations where gunmen are intent on killing as many people as possible, and often are still shooting when the police arrive on the scene.
Anderson Cooper, on assignment for 60 Minutes, begins the story from Paris.
What happened here a week ago Friday is law enforcement's worst nightmare: multiple shooters attacking multiple locations, stretching the resources of police, and crippling a city.
There was only one active shooter attack in the United States in 2000, but by 2015 there were more than one a month. They usually involve just one gunman, but American law enforcement has been expecting a Paris-style attack in the U.S. for years.
Bill Bratton: In American policing, we have no answer for why we don't have more of these events and why we don't have more that are very specifically put on by terrorist-related activists.
New York Police Commissioner Bill Bratton says the NYPD has been preparing for that kind of attack ever since the 2008 terrorist strike in Mumbai, India, that killed 173 people and shut down a city of more than 18 million for three days.
Anderson Cooper: What did you learn from Mumbai?
Bill Bratton: The idea of the-- multiple shooters consciously going in a lot of different directions.
Anderson Cooper: Multiple shooters, multiple locations.
Bill Bratton: Multiple (shooters) who had bombs in taxi cabs, railway stations, the hotel. We also learned that these people are going to take hostages only for the purpose of media attention. They're going to kill them. They're not interested in negotiating to surrender. They're negotiating just to extend the span of time that you in the media are going to cover what they're doing. So that's a very significant change where we normally try to rescue the hostages through negotiation.
Anderson Cooper: After Mumbai, you fully anticipated, we're going see that here in the United States?
Bill Bratton: That's correct.
Anderson Cooper: And you still believe that?
Bill Bratton: Still believe it. And that's why we prepare for it.
The New York Police Department is so concerned about a Paris and Mumbai-type attack, they're retraining all 35,000 police officers in the city.
[Officer: Weapon is now loaded, finger off the trigger, holster up.]
They allowed us to watch some of what they're doing. Detective Raymond McPartland is the lead trainer with the NYPD Counterterrorism Division and says its critical police move in quickly to stop an active shooter.
Raymond McPartland: The big piece I always tell people is time is of an issue for both ends. The shooter always wants more time inside because that's more victims. We need to cut his time in half, if not minimize it completely, by getting there quickly. And that's a complete shift, a paradigm change for law enforcement across the board.
Getting in there quickly means overcoming chaotic and confusing situations in pursuit of the gunman.
Raymond McPartland: So now you got a shot. So now that changes what we're doing here because we're going to go in that direction.
In this drill, a team of four officers has to stop an active shooter in a classroom full of students some of whom are already wounded.
Responding officers are told to focus on finding the gunmen, before they try to treat any casualties.
Anderson Cooper: It's also got to be tough because you have hysterical people in a classroom, they're going to be screaming, "Help this person, help this person."
Raymond McPartland: Yeah, sure, and just think of the psychological aspect. I mean, you go into a school shooting and you see children. You see-- this is something anybody's going to want to bend over and do whatever they could to stop that. But what we try to instill in them is that we need to stop the killing further.
In another training scenario, we watched the police respond to a simulated attack by two terrorists with rifles, loosely based on what happened in Mumbai.
They immediately engage in a gun battle with the first shooter who's surrounded himself with civilians.
Raymond McPartland: The issue becomes now you've got a crowded hallway. So this is how they are going to have to deal with it.
Raymond McPartland: If you notice on the floor, there is a bag. At the very least we should start thinking IED, explosive device. It's something we are concerned about.
For the first officers on the scene information is limited, and often contradictory. With every second that passes more people could be dying.
Anderson Cooper: The adrenaline is pumping so much that it changes the way you think. It changes your judgment.
Raymond McPartland: Sure. It's a survival instinct. There's a man with a gun that's in that room and he's trying to kill other people.
Anderson Cooper: Right.
Raymond McPartland: And under stress, the idea of stress science is fascinating when it comes to our world because your vision goes down to about 17 percent under stress.
Raymond McPartland: If I said "long guns," if I said "tactical gear," and I said "terrorism," what's the one thing you should also be thinking about?
Raymond McPartland: IEDs. Thinking about bombs.
Afterwards, Detective McPartland reviews the exercise with the officers and asks them about the bag that was left in the hallway.
Officer: I didn't notice the bag.
Anderson Cooper: If you had noted that that was an IED in that bag, would you still keep going for the shooter?
"People say, you know, 'What is it that keeps you awake at night?' It's not all the things that we train for and we know about. It's the one thing that we haven't yet thought about. What is it that we're missing?"
Raymond McPartland: Unfortunately, yeah. If we had to stop for every bag we found, then obviously we'd have a problem because we would never get to that guy.
A number of American cities have been retraining their police in a similar way. Washington D.C. Police Chief Cathy Lanier says their preparations have taken on new urgency since ISIS made a threat this week to launch attacks in Rome and Washington.
Cathy Lanier: People say, you know, "What is it that keeps you awake at night?" It's not all the things that we train for and we know about. It's the one thing that we haven't yet thought about. What is it that we're missing?
Anderson Cooper: We've now seen a number of people who are just ideologically motivated, who say they support ISIS, but may have no actual direct connection with a group like ISIS, but just they've watched some videos and they've decided to--
Cathy Lanier: Even scarier. Less trip--
Anderson Cooper: That's even scarier?
Cathy Lanier: Less tripwires. Less opportunity for us to intercept.
Cathy Lanier: I don't think you're going to stop the shootings. I think that a person who's committed to carrying out an act of violence like this is going to carry that act out. How successful they are and how many people they kill, we can try and intervene on.
Police departments started to take a serious look at how they respond to active shooters after the attacks at Columbine High School in 1999.
Anderson Cooper: Columbine was a real turning point, in terms of reassessing strategy in active shooter situation?
Cathy Lanier: Yes, it was huge. So, we based a lot of our training, for active shooter response, at the local law enforcement level. We based a lot of our training on Columbine.
In Columbine, two troubled teenagers freely roamed the school killing 12 students and a teacher...while outside hundreds of law enforcement personnel set up a perimeter and waited for 45 minutes before going in.
Cathy Lanier: And I very distinctly remember a parent being interviewed who said, "What were they waiting for? They have gun, my kids don't have... none of our kids had guns."
In the recent, Paris attack here at the Bataclan concert hall, police waited 35 minutes outside for the tactical team to prepare before going in. A U.S. law enforcement source described that as a "familiar old American model" that has been abandoned. Columbine taught police they had to get in fast despite the fact a SWAT team might not be there.
Cathy Lanier: This is a homicide in progress. You can't wait for backup, you can't wait for the SWAT team. You are the only thing that can stop that shooting. You have to get in there and do it.
That's what Washington D.C. police did in 2013 at the Navy Yard when a mentally-ill employee began shooting his coworkers.
[Radios: ''We have an active shooter. A male with a shotgun. Multiple shots fired. Multiple people down."]
Cathy Lanier: Our first call to 911 came in one minute and 36 seconds after the first shots (were) fired. We already had multiple people that were shot at that point.
Chief Lanier learned a number of lessons from the police response to the Navy Yard shootings. Some of the rifles police had were too big for the narrow corridors the shooter was moving through. And the sound of fire alarms made it difficult to determine where shots were being fired from.
Cathy Lanier: The flashes you see are the fire alarms. The fire alarm's been pulled. The fire alarm's going off. It's loud. And they've got gunshots being fired, and they're trying to narrow down where the gunman is so they can get to the gunman and stop the shooting.
It took police an hour and nine minutes to kill the shooter.
Anderson Cooper: And of the 12 people who were killed, the first 10 were killed how quickly?
Cathy Lanier: Six minutes.
Cathy Lanier: That fast.
According to the FBI 60 percent of active shooter attacks are over before police ever arrive, so now law enforcement agencies throughout the country are trying to educate the public on how to survive on their own
Cathy Lanier: Your options are run, hide, or fight--
Anderson Cooper: That's what you tell people they should do?
Cathy Lanier: Yes.
Cathy Lanier: What we tell them is the facts of the matter is that most active shooters kill most of the victims in 10 minutes or less, and the best police department in the country's going to be about a five-to-seven minute response.
Cathy Lanier: I always say if you can get out, getting out's your first option, your best option. If you're in a position to try and take the gunman down, to take the gunman out, it's the best option for saving lives before police can get there. And that's-- you know, that's kind of counterintuitive to what cops always tell people, right? We always tell people, "Don't-- you know, don't take action. Call 911. Don't intervene in the robbery"-- you know-- you know-- we've never told people, "Take action." It's a different-- this is a different scenario.
Anderson Cooper: You're telling them that now though?
Cathy Lanier: We are.
It is important to remember that as tragic and scary as these active shooter attacks are, it's highly unlikely you'll ever be caught up in one.
Bill Bratton: You have a very low chance of being a victim of an incident like this. But what we try to do is encourage awareness. The idea is to have an awareness without creating a fear.
Anderson Cooper: A person's chance of actually having some sort of encounter with an active shooter is, like, one in two million. A person's chance of being hit by lightning is one in 700,000. Do you worry about an overreaction? People getting too scared, fearful of something which in all likelihood they will never encounter?
Cathy Lanier: You can be prepared and you can have a society that is resilient and alert and conscientious and safer without scaring people.
Anderson Cooper: You don't want people to be afraid?
Cathy Lanier: No, that works against you. If you educate people on actions they can take to reduce their risk, then you can save some lives. And I think it's irresponsible for us not to do that. I'm not worried about an overreaction. I'm more worried about a numbness to what is potentially a reality.
Anderson Cooper: A numbness?
Cathy Lanier: Yes.
Anderson Cooper: How do you mean?
Cathy Lanier: Just ignoring it and not preparing yourself. That's not an option anymore.