BROWNSVILLE — The Zapata County sheriff Thursday was questioning why a Mexican military helicopter was hovering over homes on the Texas side of the Rio Grande.

It was one of the more jarring incidents of the fourth week of border tensions sparked by drug killings, and rumors of such killings, in the Mexican state of Tamaulipas.

Sheriff Sigifredo Gonzalez said he'd reviewed photos of the chopper flown by armed personnel Tuesday over a residential area known as Falcon Heights-Falcon Village near the binational Falcon Lake, just south of the Starr-Zapata county line. He said the helicopter appeared to have the insignia of the Mexican navy.

“It's always been said that the Mexican military does in fact ... that there have been incursions,” Gonzalez said. “But this is not New Mexico or Arizona. Here we've got a river; there's a boundary line. And then of course having Falcon Lake, Falcon Dam, it's a lot wider. It's not just a trickle of a river, it's an actual dam. You know where the boundary's at.”

The sighting came amid ongoing fighting between the Gulf Cartel and its former enforcers, Los Zetas. The mounting death toll and crisis of fear in cities across from the Texas border have drawn global attention, as has a news blackout in affected cities due to the kidnappings of eight Mexican journalists, at least one of whom was killed.

As violence continued Thursday with a highway shootout in Tamaulipas, a Senate subcommittee in Washington heard testimony that drug cartels are trying to infiltrate U.S. agencies along the border, with corruption cases among Homeland Security personnel on the rise.

In the past two years, there have been 400 public corruption cases involving federal, state and local law enforcement agents originating from the Southwest border region, Kevin Perkins, FBI assistant director for criminal investigations, told the Senate Homeland Security subcommittee on preparedness.

James Tomsheck, a U.S. Customs and Border Protection assistant commissioner, told the panel the drug cartels operating in Mexico are making a concerted effort to infiltrate CBP, and the agency is responding with more screening of job applicants with polygraph tests and background investigations. Corruption cases were opened last year on 576 CBP officers and Border Patrol agents.

In addition to the highway battle, news from Tamaulipas on Thursday included a 25-year-old man found dead on a roadside in Miguel Alemán. On Wednesday, three people died in one or two gun battles in Reynosa.

Four other deaths have been reported since Saturday in the cities of Mier, Camargo and Miguel Alemán.

Wednesday's confirmed confrontations in Reynosa included an evening run-in between elements of the ministry of defense and armed civilians and a battle between drug factions that lasted for hours. One woman was injured during a skirmish Tuesday between federal officials and drug operatives.

A military operation Wednesday in Reynosa reportedly resulted in the wounding and arrest of a man identified by witnesses as a former engineer for Pemex, the government oil monopoly. The witnesses told the Mexican newspaper El Universal that an attempt by army soldiers to stop his late-model white Suburban escalated into a pursuit with gunfire. More than 100 soldiers closed off neighborhood streets as part of the operation.

Gonzalez, the Zapata sheriff, said he couldn't confirm reports that the helicopter was scoping out the home of a drug criminal. He said the incursion about a mile over the border took place over a neighborhood populated by many U.S. Customs officers who work at area border crossings — and that they knew what they were seeing.

“My understanding is the U.S. military were informed,” he said. “I don't know what action was taken, if any.”