ZAPATA, Texas -- An American tourist was shot in the back of the head in Mexican waters on Thursday after being ambushed by armed boaters, a Texas sheriff said. It happened on a lake where run-ins with pirates had already put fishermen and Texas officials on alert.

Zapata County Sheriff Sigifredo Gonzalez said a 30-year-old man and his wife were riding jet skis back from Mexico when about six gunmen approached in two boats. Gonzalez said the man was shot as the couple sped away.

What happened to the man was unclear. Gonzalez said the man's wife tried circling back to get him, but retreated back to U.S. waters after being fired upon again.

"They saw them approaching and started revving it up back to the U.S. side," Gonzalez said. "The guys just started shooting at them from behind."

Gonzalez said he has contacted the Mexican consulate and asked them to look for the man. He said there was nothing else he could do.

The shooting happened on the Mexican side of Falcon Lake, about 60 miles down the border from Laredo. Earlier this year, several fishermen were robbed at gunpoint on the lake's Mexican side.

In those robberies, authorities say the gunmen traveled in the low-slung, underpowered commercial Mexican fishing boats that are familiar in the area. They asked for money, drugs and guns, and took what cash was available, but no one was hurt.

Falcon Lake is a dammed section of the Rio Grande that straddles the border. The border is marked by 14 partially submerged concrete towers that mark the Rio Grande's path before the lake was created in 1954.

Officials have sought in recent months to assure boaters that the U.S. side of the lake is safer.

Gonzalez said the couple shot at Thursday never spoke the gunmen. He said the couple lived in McAllen, Texas, and had ridden their jet skis over to Mexico for sightseeing.

The shooting happened about 2:45 p.m., said Texas Parks & Wildlife Department spokesman Mike Cox. He said the department has two boats with seven game wardens on the lake.

Gonzalez has previously chalked up the dangerous waters as the product of fighting between rival Mexican drug gangs.

Violence on the Mexican side of the lake has been climbing for several months, as a fractured partnership between the region's dominant Gulf Cartel and its former enforcers, the Zetas, plunged many of the area's Mexican border cities into violence.