The severed head of the lead Mexican investigator in the Falcon Lake case, Rolando Armando Flores Villegas, was delivered Tuesday in a suitcase to the Mexican military, Texas Rep. Aaron Pena told CNN.
But a spokesman for the attorney general of Tamaulipas state in Mexico said he had not heard the report.
"We have nothing official," said Ruben Dario-Rios, the spokesman. "We have not heard any report about one of our investigators being killed. We have over 1,000 investigators working for the state."
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 a U.S. citizen who disappeared during a sightseeing trip on the U.S.-Mexican border in South Texas.
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.
David Michael Hartley was last reported seen September 30. His wife, Tiffany, told authorities her husband was shot and killed during a sightseeing trip on Falcon Lake, which straddles the border.
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, 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 on the Rio Grande in Zapata and Starr counties in South Texas. The U.S.-Mexican border runs through the middle of the lake.
Rep. Henry Cuellar, D-Texas, said 60 Mexican personnel, three boats and a helicopter had participated in the search.