GWEN IFILL: Against the backdrop of high-profile tragedies in Charleston, Baltimore and other cities, additional troubling statistics have come to light, a spike in day-to-day gun violence in a number of cities across the nation. That's led to double-digit jumps in Saint Louis, in Baltimore and in Chicago, where 10 people were killed over the Fourth of July weekend.

There are as many theories of why as there are people tasked to address the problem.

We talk to them now, three of them. Stephanie Rawlings-Blake is the mayor of Baltimore, which has struggled with a surge in violence this summer that predated the high-profile death of Freddie Gray in police custody. Mitch Landrieu is the mayor of New Orleans, where shootings are down this year. And Gary Slutkin is the founder and executive director of Cure Violence, a national initiative to stop violence in 10 cities, including Chicago, New Orleans, and Baltimore.

Welcome to you all.

Gary Slutkin, we heard about the incredible rash of killings this weekend in Chicago, and you have studied it there, as well as other places. What's going on?

GARY SLUTKIN, Founder and Executive Director, Cure Violence: Well, I think what's usually missing in the conversation around violence is its epidemic nature itself, in other words, its contagious nature.

We know a lot more about violence now than we knew 10 or 15 years ago, and you never really know what gets something going. And in the U.S., now some of the cities are going up and some are not. But when it gets going, it perpetuates itself to a certain extent. And that's what's happening.

But what is most relevant is whether you can get the right things into place to cool this epidemic, this type of epidemic down. And this is being done in Baltimore in some of the neighborhoods, in New Orleans in some of the neighborhoods, in Chicago, and also in several other cities.

GWEN IFILL: I'm sorry. I'm sorry. I wanted to ask Mayor Rawlings-Blake about that, pick up the point you just made about what's happening in Baltimore.

A lot of attention on Baltimore this year, mostly having to do what some people call a riot, some call an uprising in your city streets. But this problem was there already. What's going on in your city?

MAYOR STEPHANIE RAWLINGS-BLAKE, Baltimore: The problem of violence predates me and it has been part of Baltimore's history for decades.

And Dr. Slutkin is right. It's an epidemic. It's an epidemic of gun violence. And it requires an all-hands-on-deck approach. We know that we — under my administration, we have been able to get the homicides down to the lowest number they have been since the '70s, but still — Baltimore is still a much too violent city.

And we did that by employing some of those same Safe Streets, Operation Ceasefire, community policing. We have seen success. And we have seen an uptick in homicides and violence since the unrest that we had in the city. We have also seen an uptick in arrests, and we know that we're not going to be able to arrest our way out of it. It's going to take the focus like Safe Streets, Ceasefire, the community policing, but also addressing some of these underlying issues that impact not just Baltimore, but cities across our country.

GWEN IFILL: Mayor Landrieu, you use the term culture of violence. Some people call it a culture of guns, a culture of police overreaction.

Explain to me what you mean by a culture of violence.

MAYOR MITCH LANDRIEU, New Orleans: Well, I think that we have a national epidemic in the United States of America that people walk by every day.

We're seeing the uprisings in Ferguson and Baltimore and the unsettling nature of the relationship of police and the community, which is real, by the way, and has to be fixed. But even when that's gone on across America, pretty much every day, we're going to lose 40 Americans.

Since 1980, we have lost 680,000 Americans on the streets of America to some kind of violence. And so that's what I call the culture of violence, a behavioral pattern that has developed over time that looks like when there's a minor disrespect or a beef, the way that those individuals are resolving that problem is through violence, usually at the end of a gun.

And I think it's epidemic. I think you see it all around the country. And, of course, it spikes from time to time. Statistically, we don't know why murders go up and shootings go down, or vice versa. And Dr. Slutkin can give you more information about that.

But one of the things that we know is that you can't just police your way out of this, although police are an important part of it. That's why community policing is important. That's why the interruption is important.

And if you think about it from more of a public health perspective, as well as a criminal justice perspective, you get a sense of, as Mayor Rawlings-Blake said, the all-hands-on-deck approach that we have to take to really reduce the level of violence on the street that, in my opinion, has become a cultural behavioral pattern.

GWEN IFILL: Gary Slutkin, I seem to remember having part of this conversation this time last year, when there was another wave of Fourth of July violence in Chicago. So, what's the answer to the question about what underlies this and what communities should be doing about it?

GARY SLUTKIN: Well, what the communities have at their hands now that they didn't have before is much more of, as Mayor Landrieu said, the health approach.

So now we can put into place interrupters and other types of health workers in order to treat it like we treat Ebola. And as far as Chicago goes, for example, we just had this very, very awful Fourth of July weekend in Chicago. But there were zero killings, in fact, even zero shootings, in the Englewood neighborhood, which is notorious, because a large number of health workers were trained to interrupt and to prevent spread and to keep events from happening.

The same thing happened last year on Memorial Day in four neighborhoods in Baltimore. When the city itself was having a big outbreak, the four neighborhoods in Baltimore that were using Safe Streets, which is also a Cure Violence adaptation, had zero.

And in Baltimore, there were four neighborhoods that had — two neighborhoods had over a year without a killing using the health approach. Adding this health approach is really shifting the response of the epidemic in a different way.

GWEN IFILL: Mayor Rawlings-Blake, why don't you jump in here.

MAYOR STEPHANIE RAWLINGS-BLAKE: And that's exactly what I was about to say.

Yes, we had one community that was very violent when I was coming up in Baltimore called Cherry Hill in South Baltimore. They have been over 400 days without a homicide, and that is — growing up in Baltimore, you would know that neighborhood, and that statistic would even be more remarkable.

So, I agree it's important that we expand that public health approach. The challenge is to make sure that, not just in Baltimore that we get it right, but everywhere where you take that public health approach, you have to have the public health workers trained and the right ones that can make it work.

And that's what we are doing in Baltimore with our expansion of Safe Streets to make sure that we have the right community associations, the right health — the right interrupters that are out there in our street teams that can be helpful.

We have seen progress, but, just as Mayor Landrieu is saying, we continue to see spikes in some areas. And my hope is that I will have an opportunity, as president of the Conference of Mayors, with my second vice president, Mayor Landrieu, to bring national attention to the issues that are facing our cities, so we can look at not just at the models that can help us to reduce violence, but also to develop a national agenda for cities, so we can take a look also at the resources that have really been cut off from the federal government.

The Congress has scaled back support for cities for decades, and I don't think it's a surprise that the violence and the — particularly the gun violence has had an inverse, you know, that while the investments have gone down, the violence has gone up. And my hope is that, over this year, we will develop an agenda that will help reverse that trend.

GWEN IFILL: Mayor Landrieu, I'm not sure that there is an end to this argument.

MAYOR MITCH LANDRIEU: Yes.

GWEN IFILL: But I think that the president brought it up in his remarks last week or a couple weeks ago in Charleston, and that's the roots — the root causes of gun violence. Not — the violence we're talking are shootings, not stabbings, not anything else.

Do you think that there is a specific approach that needs to be taken there? Or does that get us into a political…

MAYOR MITCH LANDRIEU: No, not — it's not necessarily that it's a political fight or not a political fight. And guns are certainly a part of it, but they're not the only answer to it.

The issue gets to be how and why our young people are resolving their differences through violence and how that happened over time. One of the things that Mayor Rawlings-Blake just mentioned ago — just the — just the amount of funding that used to come from the federal government to make sure that our national security was protected.

And national security is what happens overseas, but it's also what happens on our homeland, has been cut, for example, just for the COPS program, by 88 percent since 1966. And police departments are stretched, and when they're stretched, they don't do nearly as much community policing.

On the issue of guns, that's an important component, but if you talk to the people who are involved in this business, these young kids are telling you, hey, Mayor, it's either kill or be killed. It's tough out here.

And I'm just and Mayor Rawlings-Blake are trying to call attention to the fact that we are losing 40 lives on the streets of America every day. And what Dr. Slutkin is saying is, the shooter today is the kid that is killed tomorrow, which is why the violence interruption has to come into place.

So this has to be a holistic approach. There's a public safety approach to it. There's a public health approach to it. There's a personal responsibility approach it to. But make no mistake about it. This is clearly a national epidemic. And it's not just one city. It's all over America and it's in specific neighborhoods, which I think that, if we spent time on, we could target and do really well with and save American lives.

GWEN IFILL: Mayor Mitch Landrieu of New Orleans, Mayor Stephanie Rawlings-Blake of Baltimore, and Gary Slutkin, the founder and executive director of Cure Violence, thank you all so much.

MAYOR MITCH LANDRIEU: Thank you.

MAYOR STEPHANIE RAWLINGS-BLAKE: Thank you.

GARY SLUTKIN: Thank you, Gwen.