GEOFF BENNETT: Months of chaos and raging gang violence came to a head today in Haiti.

The prime minister, stranded in Puerto Rico and unable to return to his nation, announced late last night that he would resign as soon as a transitional government is in place and a new leader chosen.

Meantime, Kenya reportedly delayed the deployment of a police force to Haiti under a U.N. flag pending the selection of that new government.

That force would help bolster depleted and beaten Haitian security services that have been fighting heavily armed gangs for years.

But that violence has recently been made worse by an influx of more and more powerful weapons.

And, as special correspondent Marcia Biggs tells us, the weapons fueling the violence mostly come from the U.S. MARCIA BIGGS: This is where the majority of weapons in Haiti come from, Miami, Florida, but not from the giant containers leaving the Port of Miami, but from this five-mile stretch up the Miami River, where officials say they're nearly impossible to find.

We went to one of several small shipping terminals along the Miami River, which specialize in what's called breakbulk cargo.

There, we met Anthony Salisbury, the special agent in charge of homeland security investigations in Miami.

ANTHONY SALISBURY, Homeland Security Investigations, Miami: When you go to a major modernized port like the Port of Miami, you're going to see containers stacked up, very modernized, giant cranes loading them onto cargo ships, lots of access control, lots of security, formalized paperwork, declarations.

What you have here is a bunch of loosely packed goods, very informal shipping mechanism, very informal on who it's going to, what the commodity is, and how much of it is going down.

MARCIA BIGGS: Hard to police.

ANTHONY SALISBURY: Very hard to police.

MARCIA BIGGS: Wow.

This is a typical warehouse filled with packages all bound for Haiti.

ANTHONY SALISBURY: The Department of Homeland Security has some of the best interdictors in the world and the best inspectors in the world, right?

But this is daunting.

There's no way anybody can do all this.

This is one warehouse on this river.

MARCIA BIGGS: Packages are already sealed and, according to Salisbury, lacking proper documentation, since anything claimed to be worth less than $2,500 doesn't have to be declared.

ANTHONY SALISBURY: Usually a paper manifest, handwritten, you know?

MARCIA BIGGS: Right, so very bare-bones.

ANTHONY SALISBURY: And if -- potentially, if somebody didn't want that to make a manifest, very easy to make that happen.

MARCIA BIGGS: Just last year, Homeland Security announced a crackdown after Haiti's customs agency found a shipment of arms in a container labeled as church donations.

But this breakbulk cargo can only be searched one by one with mobile X-ray units.

Salisbury says it could take a month just to go through this warehouse.

ANTHONY SALISBURY: We'd have to break apart these pallets, hand-load them through the X-ray things.

MARCIA BIGGS: You would have to break apart this pallet in order to X-ray it?

ANTHONY SALISBURY: Yes, one pallet.

MARCIA BIGGS: Oh, wow.

In addition to bags of food, we saw batteries, construction equipment, even entire cars.

But, mostly, we saw sealed-up drums and cardboard boxes, packages sent by Haitian Americans to families back in Haiti, where poverty and hunger are rampant.

ANTHONY SALISBURY: There is a humanitarian need knowing what the people of Haiti are dealing with right now.

And the Department of Homeland Security has to balance that on the whole.

How do we balance inspection and enforcement, while not unnecessarily slowing down the flow of commerce?

But can you hide guns on that?

Sure.

MARCIA BIGGS: A recent U.N. report states that the principal source of guns in Haiti is Florida, guns bought through individual or straw man purchases in U.S. states where gun laws are lax.

Handguns sold at Florida gun shows for $400 to $500 draw as much as 20 times more in Haiti.

Are you seeing any sort of thread among these straw purchasers?

Is there any indication that there may be some sort of organized, centralized structure that is leading this?

ANTHONY SALISBURY: Yes, absolutely.

Our entire goal is always to look for the network behind the individual gun, the individual kilo, whatever it is.

That's what HSI's goal is, is to identify the entire criminal network surrounding that illicit movement.

We have increased our partnerships with our international operations and our foreign partner counterparts, where these weapons are heading.

MARCIA BIGGS: While law enforcement officials are focused on gun trafficking along the Miami River, members of Congress are focused on new legislation to determine who's actually buying those guns.

REP. SHEILA CHERFILUS-MCCORMICK (D-FL): When it comes to Haiti, we know gangs, the gang members, if you look at them, some of them aren't wearing shoes.

Their clothes are ripped.

They have blood from the day before.

These aren't people who can afford to do this.

These people are actually hired.

They are trained.

They're given guns, and they're carrying out a mission.

The real question is, what is the mission?

Who is benefiting from instability?

MARCIA BIGGS: Before this latest round of violence, we sat down with Florida Democratic Congresswoman and Haitian-American Sheila Cherfilus-McCormick.

She's co-sponsored the Haiti Criminal Collusion Transparency Act, now on its way to the Senate.

It requires the State Department to present annual reports to Congress on ties between criminal gangs and political and economic elites in Haiti, information it doesn't routinely share.

REP. SHEILA CHERFILUS-MCCORMICK: The State Department would have to report to the public any actors who participated either funding the gangs contributing to the gang, sending guns, so we can all know who these people are and be able to levy sanctions against them.

MARCIA BIGGS: In addition to sanctions, last year, the U.N. authorized a multinational security support mission in Haiti to be led by Kenyan forces with funding from the U.S. After a deal was finally reached last week to send 1,000 police officers, today, Kenya put a hold on the deal until a new government is formed.

And back in the U.S., Congress has only released a small portion of the funding.

Yesterday, in light of the recent violence, Cherfilus-McCormick reiterated support for the mission.

REP. SHEILA CHERFILUS-MCCORMICK: Every day that we are waiting on appropriate funding, every moment that we're waiting on this mission is every single day that we are actively losing lives of Haitian people and Americans who are in Haiti.

MARCIA BIGGS: But the mission has raised eyebrows among those familiar with Haiti's history and dark legacy of foreign intervention.

The U.S. led a brutal occupation of Haiti in 1915 that lasted for almost two decades.

And, in 2010, U.N. peacekeepers brought cholera to the country, which killed almost 10,000 people and remains an epidemic to this day.

Cherfilus-McCormick is the only Haitian-American member of Congress, and her district has one of the highest Haitian-American populations in the country.

For her, this is personal.

REP. SHEILA CHERFILUS-MCCORMICK: We have to stop the torture.

We have to stop kids from being recruited every single day.

This is why I got elected.

The reason why the Haitian people came together and said we need to have a Haitian-American in Congress is for this time and this moment for me to speak against people who are saying, no, don't intervene, let Haitians die.

MARCIA BIGGS: A major lifeline has been the humanitarian parole program, which allows vetted Haitians to legally live and work in the U.S. for two years.

It's come under attack by Republicans who last month blocked a bipartisan border security bill.

REP. SHEILA CHERFILUS-MCCORMICK: There are no visas coming out of Haiti.

The only way out of Haiti right now is the parole program.

If you remove the parole program, that is a death sentence for every Haitian living in Haiti, eight million people we will condemn to a death sentence because you cannot leave.

The only way you would get out is on the boats when you come to my borders in Miami.

And I have gone to the detention centers.

I look them in their eyes.

I have seen their pain, or I have seen them at the border.

So how do we tell them no?

I'm sorry.

MARCIA BIGGS: While violence rages and humanitarian disaster looms, a pipeline to Haiti is vital for so many in need.

But it's also a perfect way for gun smugglers to move the weapons that keep the conflict going.

So this is where all those items that we just saw are going to end up, on a cargo hold on its way to Haiti.

You can see how impossible it would be to investigate all these items.

Any one of them could be filled with guns.

PHILOME CHARLES, God is Able Shipping: If you have a gun, weapon inside, $1 million, U.S. dollar, and you got 20 years in jail.

MARCIA BIGGS: But shipper Philome Charles says his company, God Is Able Shipping, has been sending packages to Haiti for the last decade.

And his customers know not to send weapons.

PHILOME CHARLES: If you have a gun to send it to Haiti, keep it for yourself, no God Is Able.

MARCIA BIGGS: While agents do make random inspections, often with mobile X-rays, Charles says they have never found anything.

He says it's because he documents every package that comes in, and he trusts his system.

So it looks very informal to us, to our eye, but what you're telling me is that you have information on each and every package, you know who is sending what?

PHILOME CHARLES: Yes.

You're the customer.

You bring the boxes to send it to Haiti for your family.

First, I'm going to ask you your driver license, and, second, your passport with a U.S. visa, how you come into the United States.

If you do find something, we already know who bring it to me.

ANTHONY SALISBURY: Well, like I said, relationships with you guys are important.

MARCIA BIGGS: Originally Haitian, Charles has lived in the U.S. for 38 years.

And he says he has an allegiance to both countries.

PHILOME CHARLES: I have to protect both my business, United States and Haiti, my reputation.

So far, I'm happy.

So far, I'm -- sleep good.

MARCIA BIGGS: But do you worry about those stories giving people like yourself a bad name?

PHILOME CHARLES: We make sure we (INAUDIBLE) every piece of paper for each customer.

Every mistake they have in the paper, we're not going to send nothing for you.

MARCIA BIGGS: Yet, somehow, guns make it through.

Whether it's the flow of weapons or the power of the gangs who wield them, shutting down either seems next to impossible.

And it is the Haitians who suffer.

For the "PBS NewsHour," I'm Marcia Biggs in Miami, Florida.