(CNN) -- The lead Mexican investigator in the Falcon Lake case, Rolando Armando Flores Villegas, has been killed, his severed head delivered Tuesday in a suitcase to the Mexican military, officials told CNN.

"His head was delivered to the army garrison this morning in a suitcase after he failed to report back home last night," Zapata County, Texas, Sheriff Sigifredo Gonzalez Jr. said.

A spokesman for the attorney general of Tamaulipas state in Mexico, Ruben Dario-Rios, confirmed the killing Tuesday afternoon in a telephone interview.

The report came a day after authorities in the Tamaulipas state attorney general's office gave conflicting information on whether authorities were pursuing a pair of suspects in the case of David Michael Hartley's disappearance.

Hartley's wife, Tiffany, told authorities that her husband was fatally shot September 30 during a sightseeing trip the two were taking on Falcon Lake, which straddles the border.

While Luis Homero Uvalle, a spokesman for the office, told CNN the suspects are brothers who are "well known to this area" -- identifying them only as "El 27" and "El 31" -- Dario-Rios, the chief spokesman for the attorney general said, "We have nothing official about suspects in the disappearance of David Hartley. I do not know where that is coming from."

Dario-Rios said Monday that Flores had not indicated to him that any suspects had been identified.

Eva Rodriguez, Flores' secretary, said the news of his death came as a surprise. "We saw [Rolando] last night," she said. "After he came back [from the search for Hartley] we were all together here in the office. That was the last time any of us saw him."

She said she was not aware that Flores -- whom she described as "very dedicated to his job" -- had received any threats from narcotraffickers.

"What can you do?" she asked. "We're still going to be here. We still have to work."

And, despite the fact that "we're all scared right now," their work will go on, she vowed. "The investigation and search will still continue," she said.

On Monday, Tiffany Hartley, along with David Hartley's mother, Pam, appeared on television talk shows asking for information to help investigators find those responsible for David's death and find his body.

"Until we have him back, it's not final," Tiffany Hartley said on NBC's "The Today Show."

On Sunday, the U.S. Border Patrol, the Coast Guard, and Parks and Wildlife officials were back on the U.S. side of the lake searching for new evidence in the case, said Zapata County Sheriff Sigifredo Gonzales.

Authorities from both nations have been conducting separate searches and holding regular meetings, U.S. State Department spokeswoman Virginia Staab said. But because the disappearance allegedly occurred on the Mexican side of the border, the United States cannot prosecute or make arrests in the case, the sheriff said.

Falcon Lake is a reservoir on the Rio Grande. The U.S.-Mexican border runs through the middle of the lake, with Zapata and Starr counties on the U.S. side.

Rep. Cuellar said 60 Mexican personnel, three boats and a helicopter had participated in the search.

CNN's Divina Mims, Eduardo Aragon, Ed Lavandera and Nick Valencia contributed to this report.