Nearly three months after American David Hartley was shot and killed by a drug gang while jet-skiing with his wife on a lake straddling the Texas-Mexican border, U.S. investigators say the gunmen were "authorized" by their cartel to kill Hartley's wife, as well.

David Hartley, 30, of Milliken, Colo., was shot in the back of his head on Sept. 30 by alleged Mexican drug smugglers while jet-skiing on Lake Falcon with his wife, Tiffany.

Tiffany Hartley, 29, told authorities that the couple had set out on their jet skis to photograph the Old Guerrero church on the Mexican side of the lake -- an area notorious for illicit drug smuggling. Hartley said her husband was shot when the two tried to dodge bullets from three speed boats driven by the alleged smugglers. Hartley's body has never been recovered.

Mexican authorities in October identified two suspects -- Juan Pedro Saldivar Farias and his brother Jose Manuel, both members of Mexico's Zeta drug cartel.

On Wednesday, Zapata County Sheriff Sigifredo Gonzalez said he recently received information on two additional suspects who may have disposed of Hartley's body and jet ski. He declined to name them, saying, "We don’t want to alert them that we know who's involved."

Gonzalez, citing confidential informants, also said that suspected Zeta drug cartel members on board the three speed boats were also ordered to kill Tiffany Hartley, who managed to escape. 

"Information has been revealed that authorized her killing as well, but she was able to escape," Gonzalez said. "They were targeted because they were in the wrong place at the wrong time."

Gonzalez also said that he believes at least six drug cartel members – possibly eight – were on the lake when the shooting occurred.

A U.S. law enforcement source speaking to FoxNews.com on condition of anonymity said Hartley's body and jet ski were likely destroyed to hide any evidence.

Gonzalez expressed frustration with Mexican law enforcement, saying sometimes "they just won’t call back," but said he’s optimistic the killers will eventually be caught and brought to justice.

"Eventually something’s going to happen,” he said, “Information is constantly flowing.”

Farias and Manuel are suspects in a string of murders, kidnappings, armed robberies and fires in the northern Tamaulipas state where the shooting took place.

Both suspects are reportedly from New Guerrero, Mexico -- close to the Old Guerrero church that Hartley and her husband had visited.