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"The truck Hartley and his wife used to put their Jet Skis in the water at the lake had Tamaulipas state plates, and the Hartleys drove the Jet Skis to the Old Guerrero area of the lake, a known battleground in the ongoing war the Los Zetas and Gulf cartels," according to STRATFOR, a Texas-based think tank on intelligence and international issues.

"Given the couple’s license plate and method and direction of travel, it is possible that Zetas scouts identified them as a Gulf cartel surveillance team," STRATFOR said in its report.

Gonzalez said the killing was intended as "a message to Mexico to back off, for the search to stop." But while searchers returned to the 60-mile-long lake Wednesday, the sheriff said the prospects for finding Hartley's body are fading.