David Hartley Search Goes Forward, Despite Grisly Killing of Mexican Investigator

ZAPATA, Texas (CBS) The murder of Mexico's lead investigator, whose severed head was sent to that country's military, was a stark setback in the already difficult search for missing American David Hartley.

PICTURES: Tiffany and David Hartley

However, both U.S. and Mexican authorities say they are using the killing as motivation, and plan to push forward to determine exactly what happened the day Hartley went missing in a lake straddling the border.

"We continue the search on this side of the border," Zapata County, Texas, Sheriff Sigifredo Gonzalez Jr. told CNN. "We're also sending a message that we're here."

Despite their determination, CNN reports that neither U.S. nor Mexican authorities are any closer to bringing home Hartley's body.

"Our investigators are out there right now. A body has still not been found," said Ruben Dario-Rios, a spokesman for the attorney general of Tamaulipas state.

Hartley's wife, Tiffany, told police that her husband was shot Sept. 30 while they were sightseeing on Falcon Lake, a reservoir on the Rio Grande.

Investigators believe the gunmen may be linked to a Mexican drug cartel, and opened fire because they mistook Hartley for a rival gang member.

Tiffany believes that her husband is dead, but continues to plead with whoever is responsible.

"We just want David back," she told CNN. "Maybe they can't provide a body, I don't know. But they can provide something of evidence...We just want him back. Give us that, and we'll go away."

COMPLETE COVERAGE OF THE FALCON LAKE SHOOTING ON CRIMESIDER