For a few pesos, boatmen along this jade-hued jungle river will take people or cargo across, no questions asked. On one recent day, rustlers could be seen driving long-horned cattle from trucks at river's edge onto waiting boats.

That's just garden-variety smuggling. Of greater concern are the tons of illicit narcotics that move north, and the drug cartel gunmen who move easily in either direction, committing crimes on one side only to escape to refuge on the other.

Two weeks ago, assailants thought to be from Los Zetas, a Mexico-based criminal cartel, stormed a ranch in Guatemala's Peten region and killed 27 people, beheading most of them. Guatemala's army raced to cut them off before they could get back across the border, but failed.

The lack of security along Mexico's border with Guatemala is triggering concerns in Central America and as far away as Washington. Authorities now think that three Mexican drug groups have moved into the Peten, where they operate virtually unchallenged. Guatemala's president, Alvaro Colom, is voicing alarm.

"They are invading us," Colom told El Pais, a Madrid daily newspaper, in an interview last week. "And either the countries of Central America join together to fight them or they will defeat us and finish off our democracies."

Worse yet, there's evidence that Los Zetas are using the lack of security on the border to smuggle north deserters from a feared Guatemalan army unit known as the Kaibiles to serve as ground troops and enforcers in Mexico's bloody drug conflict.

U.S. officials think that Mexico is ill-equipped to respond to the situation. Mexico has massed most of its army in the north, near the U.S. border, to counter the narcotics traffickers who've killed thousands there as they battle for lucrative smuggling routes.

"The last thing they want to do is open up another front in the south before they're able to get their arms around the challenges in the northeast," Adm. James Winnefeld Jr., the head of the U.S. Northern Command, the military district that includes Mexico, told the Senate Armed Services Committee last month, referring to Mexico's government.

The Kaibiles began counterinsurgency operations in the 1970s as a civil war gripped Guatemala, and practiced scorched-earth tactics against Mayan villages suspected of harboring insurgents. The Kaibiles slogan is: "If I advance, follow me. If I stop, urge me on. If I retreat, kill me."

A U.S. diplomatic cable obtained by WikiLeaks highlighted how few resources are devoted to policing Mexico's southern border. In the cable, made public last December and written 11 months earlier, a U.S. political counselor wrote that 30,000 U.S. Customs and Border Protection agents keep track of the 1,926 mile U.S.-Mexico border.

But on Mexico's southern frontier, he wrote, "only 125 Mexican immigration officials monitor the 577-mile border with Guatemala. Mexican immigration officials repeatedly confirmed that they do not have the manpower or resources to direct efforts effectively along the southern border."

The military presence on the Mexican side is also slight. During the 1990s, when Mexico faced a leftist insurrection in Chiapas, its southernmost state, some 40,000 soldiers were based in that state alone. Today, just 14,000 soldiers patrol in Chiapas and Tabasco, another state, which with Campeche and Quintana Roo makes up the rest of the Mexican side of the border.

The border with Guatemala has eight official crossing points, only three of them with significant traffic, mostly near the Pacific end of the border.

But it's the unofficial crossing points - perhaps as many as 44 - that carry the bulk of the traffic between the countries. Some of those are marked with gates and chains operated by local peasants, who collect a "toll" but don't care what merchandise passes through. Many are river crossings like those along the Usumacinta River, which forms a portion of the border with Chiapas state.

"There are areas where it is easy to pass across," said Col. Rony Urizar, a spokesman for the Guatemalan army. "There are crossings that are not protected."

On the face of it, few crossing points could be more tranquil than Frontera Corozal, which sits on the Mexican side of the Usumacinta, a name that means River of the Sacred Monkey. Local officials feigned no knowledge of the massacre in Guatemala, even though it was the trigger for a 30-day state of siege that Colom imposed on the northernmost third of his country.

"We didn't know anything about this," said Raul Arcos Mendano, an ethnic Chol who's the municipal agent, a post similar to a mayor. "There's no alert here."

Lawlessness along the border is common.

Townspeople said that some immigration officials profited by demanding bribes from undocumented migrants arriving from Central America. As many as 300,000 undocumented migrants cross into Mexico each year, and the United Nations estimates that the illegal industry to control their passage nets $6.6 billion annually.

That's given Los Zeta another lucrative industry, people smuggling, in which they collude with corrupt local officials, townspeople say.

That collusion led to a purge earlier this month of immigration chiefs in seven of Mexico's 31 states. The purge followed charges by Central American migrants passing through Tamaulipas state, near the U.S., that agents had pulled them off buses and handed them over to criminal gangs.

While the overlap between drug and migrant smuggling is worrisome, what really unnerves law enforcement officials is the impunity with which the drug gangs are able to operate in both countries, bringing to Guatemala the violence that pits the Zetas and the vast Sinaloa Cartel against Mexico's Gulf Cartel.

A banner hung in Quetzaltenango and signed by a Zetas group, calling itself "Z 200," took responsibility for the Peten massacre, saying the vanished owner of the ranch, Otto Salguero, "is one of the most important suppliers of cocaine to the Gulf (Cartel) and those who paid with their lives are employees of his who maintain his organization."

On the other side of the border, the governor of Tabasco state, Andres Granier, told reporters that deserters from the Kaibiles were responsible for the killings May 17 of 10 people in a repair shop in the city of Cardenas, one of the fronts in a turf war between Zetas and the Gulf Cartel.

The former Kaibiles were armed with grenade launchers capable of knocking down helicopters, Granier said.