One marine was killed in the shootout, which occurred Sunday afternoon on a portion of the lake some two miles northeast of Nueva Ciudad Guerrero, part of the Zeta stronghold in northwest Tamaulipas state.

"Sadly, one member of this institution lost his life in completing this mission," the navy said in a press release.

The island was believed to be a holding place for marijuana carried on speedboats to Texas shores, the navy said. Following the battle, the military seized a cache of firearms, including 19 large arms, a .50-caliber rifle and a machine gun as well as ammunition, bulletproof vests and surveillance equipment.

The news came on the eve of President Barack Obama's visit to El Paso today to make a push for comprehensive immigration reform. Republicans have repeatedly said they won't support immigration reform until more is done to secure the border.

Warnings of 'pirates'

The Mexican side of the binational reservoir has become notorious as the site where David Hartley, a U.S. citizen, was presumably shot on Sept. 30, 2010. His body was never recovered.

U.S. authorities had previously warned of "pirates" shaking down tourists and fishermen in Mexican waters and continue to urge visitors to remain in Texas waters.

Hartley's wife, Tiffany, said the two were ambushed near the Mexican side the lake. She raced her personal watercraft back to the U.S. after the incident, and has since become an outspoken advocate for border security.

On Monday Tiffany Hartley was en route to Washington to attend a congressional hearing.

The Texas side of the lake straddles largely remote Starr and Zapata counties, where dense brush lining the shores provides ample cover for drug smugglers.

Zapata County Sheriff Sigifredo Gonzalez, the main U.S. investigator in the Hartley shooting, said he was angry he wasn't notified of the gunfight.

U.S. sheriff is upset

"We have gotten basically no information whatsoever from Mexico," he said. "The way I learned of this incident … was through media."

Gonzalez said sources told him Monday that the Zetas who weren't killed in the attack commandeered nearby commercial fishing boats and fled, likely to U.S. shores.

"It was Mother's Day yesterday. There were people out there (on Falcon Lake), perhaps barbecuing," he said.

U.S. authorities said they lacked jurisdiction to search the Mexican side, and Tamaulipas investigator Rolando Flores, who'd promised support to the Hartley family, was killed, his decapitated head delivered to an army post in a suitcase.

Conditions have deteriorated in the months since, as the Zetas try to maintain control over the small Mexican towns opposite the Texas border between Laredo and McAllen.

Attacks, mass graves

In late April, cartel gunmen attacked the cities of Miguel Alemán and Ciudad Mier, torching buildings and disrupting life just months after a siege that forced the evacuation of Mier.

In San Fernando, to the southeast, another Zeta stronghold, authorities spent much of April digging up mass graves. The latest tally is 183 bodies, believed to be uncooperative recruits taken in an attempt to bolster depleted Zeta ranks. Many show signs of being bludgeoned to death.

The military, and in particular the small and somewhat elite navy, has appeared to make inroads under Mexican President Felipe Calderón, most recently announcing the capture of Martín Omar Estrada Luna, known as "El Kilo," a U.S.-raised leader of the Zeta cell blamed for the mass graves.

The road to San Fernando, dubbed the "highway of death" because of the mass graves, is now dotted with military checkpoints, where soldiers give travelers an emergency number to call should they be pursued.

lbrezosky@express-news.net