A shooting incident last month that forced a U.S. border patrol helicopter to make an emergency landing near Laredo, Texas, was the work of Mexican drug traffickers, and analysts say the attack highlights growing narcotics trafficking across porous U.S. borders.

According to U.S. officials familiar with an investigation of the June 5 incident, members of the Los Zetas drug cartel were crossing back into Mexico from the United States when they were spotted by a U.S. Customs and Border Protection (USCBP) helicopter along the Rio Grande River near Laredo.

The traffickers had finished delivering a shipment of drugs and were returning to Mexico when they were spotted by U.S. agents and opened fired with automatic weapons.

The helicopter, part of USCBP’s Office of Air and Marine, was struck by gunfire on its side and on the rotor blade. The pilot was forced to make an emergency landing.

The law enforcement officers on the helicopter spotted the traffickers along the river during a routine flight around 5:00 P.M. local time June 5.

"The pilot was able to make a safe landing; there were no injuries," said USCBP spokesman Daniel Hetlage, adding that U.S. and Mexican authorities are continuing to investigate. He declined to elaborate.

"I understand that they were chasing some people with bundles of marijuana," Webb County Sheriff Martin Cuellar told the Laredo Morning Times. "People are getting desperate and crossing narcotics across the border."

The helicopter that took fire was an EC-120, a medium-range turbine engine-powered aircraft.

A U.S. official said the helicopter attack was unusual but not unprecedented. The incident was not widely reported at the time and highlights the increasing danger of porous U.S. borders and widespread drug trafficking that takes place across them, the official said.

U.S. border security problems are expected to be a major topic of debate during the presidential election campaign.

The area near Laredo is a major transit route for Zetas drug runners.

Joel Vargas, head of intelligence for the international association InterPort Police, said the recent escape of Mexican drug kingpin El Chapo Guzman will re-energize drug cartel cells in Mexico that are battling the major Sinaloa drug cartel.

"The partnership between the Gulf Cartel and Los Zetas, even with their own internal fighting going on, makes the border town of Laredo, Texas, a powder keg," Vargas said. "El Chapo will re-attempt to take back not only Laredo, Texas, but also consolidate control of El Paso, Texas."

A month after the U.S. helicopter was forced down by gunfire, Mexican authorities killed six drug runners near Mexico’s Nuevo Laredo, across the border from Laredo, Texas.

The six suspects had fired on a Mexican Blackhawk helicopter, hitting it several times.

The Mexican helicopter incident July 6 involved an armed convoy of suspected Zetas drug cartel members.

According to U.S. officials, the Zetas are a well-armed organization. Authorities in Guatemala have captured M-16 and AK-47 rifles and grenades from Zetas operating in that country.

The Zetas also make extensive use of social media. The group has posted photos of beheadings it has carried out against members of rival drug gangs. It has also claimed responsibility for killing several bloggers who they say had exposed some of the group’s members.

The Zetas were implicated in an Iranian plot in 2011 to kill the Saudi ambassador to the United States, Adel al-Jubeir.

Iran’s Islamic Revolutionary Guards Quds Force, a paramilitary and covert action force, attempted to recruit Zetas members to conduct attacks against the United States.

The Quds force also has been seeking to collaborate with Zetas in setting up transit routes that will be used to smuggle Afghan heroin into the country.

As a result of the 2011 plot, the Obama administration placed Quds Force commander Gen. Qasem Soleimani, on the list of designated terrorists.

The Iran nuclear agreement reached in Vienna earlier this included Soleimani on a list of Iranians who would have sanctions against them lifted in the future.