Were American tourists caught in the crossfire of a drug war? Or is there a more sinister twist to the story? Don't miss "Murder in Mexico: What Happened at Falcon Lake" Saturday, April 16, at 8 p.m. ET on CNN.
(CNN) -- It was planned as a day of recreation for the young couple but turned into tragedy. A 30-year-old man dead, his body lost -- possibly forever.
As his grieving widow waits for answers, suspicion is cast on her. Then an investigator is murdered as he looks into the crime.
Today, nearly six months later, some of the puzzle pieces are falling into place regarding what happened to Americans David and Tiffany Hartley that September day on Falcon Lake, which straddles the U.S.-Mexico border.
Even with the new information, however, authorities caution that the full picture of what happened that day, leading to David Hartley's death, may never be known.
"It didn't happen in the United States," said Zapata County, Texas, Sheriff Sigifredo Gonzalez.
And Mexican authorities, he told CNN, have "somewhat of a zero solvency rate, and a zero conviction rate."
"So unfortunately, this case may remain open forever, even though the information and the evidence may be there," according to Gonzalez.
A suspicious story
Tiffany Hartley has told police that she and her husband were on Sea-Doo personal watercraft on the lake on September 30, to visit a half-submerged church, when they were ambushed by assailants who shot David Hartley in the head.
She said she was unable to haul his body onto her watercraft before being forced to flee.
Early on, authorities surmised the couple had stumbled into a drug transaction, as the area is widely known as a Mexican drug cartel's stomping ground.
Tiffany Hartley's story was greeted with skepticism by some who believed that something more sinister was at work.
But Gonzalez's support of her never wavered.
"I still believe Tiffany," he said.
Gonzalez says what she told him in interviews after the shooting called to mind two similar incidents in April and May of last year -- incidents she had no way of knowing about.
Before David Hartley's killing, there were four reports of American fishermen running into Mexican pirates on Falcon Lake, authorities said. In each case, the fishermen were warned to stay off the Mexican side of the lake and sent on their way.
"Everything she's telling me has happened before in the other events, the other cases," Gonzalez said.
Doubts about Tiffany Hartley's story were somewhat lifted after the gruesome murder of Rolando Armando Flores Villegas, who was leading the Mexican investigation into her husband's death. His head was delivered in a suitcase to a Mexican military post shortly after David Hartley's death.
Gonzalez said he doesn't believe that the Hartleys were involved with drugs or that Tiffany Hartley was somehow involved in her husband's death to collect insurance money -- some of the theories that have been bandied about by those who doubt her story.
"There is no evidence to indicate any of that," he said.
Wrong place, wrong time
The new information, he said, seems to support investigators' theory that drug cartel members killed Hartley because the couple was seen as a threat.
Hartley could merely have fallen victim to an unlucky shot from assailants who intended to disable, not kill, according to Gonzalez and another expert.
"They opened up with automatic weapons or semiautomatic weapons and just started throwing rounds in the direction of the Hartleys," said Fred Burton, vice president of the global intelligence agency Stratfor.
"[By] sheer volume ... [the gunmen] could get lucky and hit your target."
A few informants have come forward in the case, Gonzalez said, although obviously, they fear for their safety.
One says he was told what transpired by those involved in the shooting. Another claimed they were on the water at the time the shooting occurred, and heard what they described as a "war zone" -- even from a couple of miles away.
The couple's decision to use their personal watercraft on the lake where there had been several attacks on fishermen was "stupid," according to local fishing guide and resort owner "Speedy" Collett.
Collett says fishermen are completely safe on Falcon Lake as long as they follow the unwritten rules. Nevertheless, he says business has been down since Hartley's killing in September -- something Gonzalez echoed.
Tiffany Hartley admits she knew "there were attacks" on the lake, but she says "we didn't know where they were exactly."
"We hadn't heard anything for a while, and we were just there in August and enjoyed three, four hours that day on the [lake]."
Investigators believe there were seven gunmen on three boats, according to Gonzalez, who said it's his understanding that the cartel was "getting ready to cross a drug load" and saw the Hartleys as a threat.
The instruction was given to "go ahead and shoot at them," he said.
Parts of Falcon Lake are under direct control of the Zetas drug cartel, Burton said. The area is notorious for the moving or storage of thousands of pounds of marijuana, Gonzalez said.
"This was a very strong smuggling corridor for them," Gonzalez said. "Therefore, anybody that's an outsider that goes into that area is viewed as either working for another cartel or a possible informant for a government agency."
The Zetas police the area, Burton said, and are responsible for surveillance. Tiffany Hartley last year recounted for CNN seeing her husband shot, and the gunmen returning as she struggled to save him.
She said she was holding her husband by his life vest with one hand and her Sea-Doo with the other when she found herself staring down a gun barrel. She said she looked the two men in the eye and pleaded with them not to shoot her.
After a whispered conversation in Spanish, the two left, she said, and she took off on her Sea-Doo.
More shots rang out, she said, but she didn't look back.
New details emerge
Gonzalez said the first informant, who heard about the shooting from those involved, also said that Tiffany Hartley was briefly held at gunpoint.
The gunmen did fire at her as she fled, the informant said, but were not able to hit her because "she kept zigzagging all over the place."
Burton believes in a slightly different version of events, speculating there was a moment where the people involved realized they had made a mistake, and let Tiffany Hartley go after realizing she was not a threat.
"From all the individuals that I've spoken to that are very close to the investigation, not only inside the United States but in Mexico, they pinpoint nothing but a story of just a couple being in the wrong place at the wrong time," Burton said.
As for David Hartley's body, Gonzalez and Burton agree it will probably never be found.
Gonzalez said informants have told him how it was disposed of, but would not elaborate because he lacks concrete information.
The Sea-Doo, he said, was also destroyed.
"That body is evidence that they really don't want to come to light," Burton said.
Mexican authorities have identified two suspects in the case, and the U.S. has identified two more, Gonzalez said, leaving three others unidentified.
The sheriff previously has said that at least some suspects may be dead. Burton believes they were killed by the Zetas.
"The killers were killed by the organization because, remember, this is bad for business," Burton said.
Like Gonzalez, he is skeptical that the Mexican investigation will bear fruit.
"There's so many homicides inside of Mexico that are unsolved that this is just another one," he said. "... I think politically, the Mexican government looks at this kind of case and says, 'We have thousands of unsolved homicides. Why is this one so important?' "
Some may find that shocking, he said, but "I think most people fail to recognize the scope of violence that we have seen over the past 18 to 24 months around the border."
"I think Mrs. Hartley is in a very difficult situation in that the likelihood of the Mexican government ever successfully resolving this case to where there's an arrest or prosecution of the people involved is highly remote," Burton said.
"If the Zetas have killed the people that are responsible for the crime, that's all the justice Mrs. Hartley could probably expect at this point in time."
Tiffany Hartley, meanwhile, said she is now thinking of starting a nonprofit organization "to fight for America and our borders."
She believes her husband was a victim "of terrorism, of politics and the way of life in Mexico," adding that she believes too much money flows back and forth across the border.
And she believes her husband's body, one day, will come home.
"God's word says, 'Whatever has been stolen from you will be given back to you a hundredfold,' " she said. "And I believe, a hundredfold, I will get everything that's been stolen from me. And that's my husband."
CNN's Ann O'Neill contributed to this report.