Ruben Rios, a spokesman for the Tamaulipas state prosecutor’s office, said Rolando Flores, the commander of state investigators in Ciudad Miguel Aleman who was part of a group investigating the reported shooting of David Hartley, was slain.

Rios said authorities “don’t know how or why he was killed. We don’t have any details on how he died.”

But Zapata County Sheriff Sigifredo Gonzalez told The Associated Press later Tuesday that “reliable sources within the law enforcement community of Mexico” told him that Flores’ head was found Tuesday morning in a suitcase outside of an Army base.

Cartels have used beheadings to terrorize the public and send messages to Mexican law enforcement. U.S. officials have said threats from drug gangs who control the area around Falcon Lake have hampered the search for Hartley.

Gov. Rick Perry said Tuesday that backing off when confronted by threats like Flores’ slaying is “the worst thing we can do.”

“I think their attempt is to intimidate law enforcement, no matter who they are or where they are,” Perry told the AP. Their message is to “stay out of their territory.”

“The worst thing we can do is let the terrorists dictate the terms of how we’re going to live.”

Perry said the threat should be handled by increasing “the numbers of law enforcement and military.”

Hartley’s wife, Tiffany, said she and her husband were on Jet Skis on Sept. 30, returning to the U.S. from photographing a half-submerged church in Mexico, when they were attacked by pirates on speedboats. Hartley was shot and presumably fell into the lake. Tiffany Hartley said she tried to retrieve her husband’s body and his Jet Ski but the pirates continued firing and she fled. Gonzalez has said he has an eyewitness who corroborates her account.

U.S. officials, particularly Perry, and Hartley’s family have been pressuring Mexico to step up the search for Hartley and determine what happened.

Gonzalez said Mexican divers searched the lake Monday and Tuesday, and that the chances of finding Hartley’s body grow slimmer with each passing day.

“The possibility is there. But in all probability the body will never be found,” he said.

Falcon Lake is a dammed section of the Rio Grande, 25 miles long and 3 miles across. Pirates have robbed boaters and fisherman on the Mexican side, prompting warnings to Americans by Texas state officials, but Hartley’s death would mark the first violent fatality on the lake.

Dennis Hartley, David Hartley’s father, expressed shock and regret at Flores’ killing.

“I just, I’m in shock about this right now,” he told The Associated Press from his Colorado home. “I really don’t have any hope that David will be found. I really hate other people putting their lives at stake. We don’t need more sons lost. If this is true, I’m just really heart broken that this happened.”

The Mexican Foreign Ministry says it has been using federal, state and local resources, including the military and helicopters, to search for Hartley’s body and opened an investigation. Over the weekend, authorities named two possible suspects.

That part of Tamaulipas state is overrun by violence from a turf battle between the Gulf Cartel and the Zeta drug gang, made up of former Mexican special forces soldiers, and both are battling the Mexican military.

Last week, Perry had asked Mexican President Felipe Calderon to call him within 48 hours and said that he expected a body to have been recovered by then. Perry also said last week that even the threat of drug gang violence against search crews was no reason to halt the efforts.

Meanwhile, Rios said no suspects have been identified in the case and said he wouldn’t comment on why a state investigator had already named two suspects. On Sunday, state investigator Juan Carlos Ballesteros, who is assigned to Ciudad Miguel Aleman, said police believe brothers Juan Pedro and Jose Manuel Zaldivar Farias may have killed Hartley. Ballesteros didn’t answer calls seeking comment Tuesday.

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Rodriguez reported from Mexico City. Associated Press Writer P. Solomon Banda contributed to this report from Denver.