Oct. 13, 2010  -- Mexican authorities have said they will press their search for the body of an American tourist even after the lead investigator was decapitated.

"I still believe they are going to do what they can for David and for us, for bringing him home," said Tiffany Hartley, whose husband, David Hartley, was believed to be killed by Mexican pirates while out sightseeing on a border lake near Texas.

Hartley said she had met last week with Rolando Flores, the head of state investigators in Ciudad Miguel Aleman, and that he seemed to be working hard to find her husband.

"I can't even explain how my heart is just broken for him and his family. There aren't any words," she told "Good Morning America" today. "He was just such a nice guy and he really wanted to do what he could for us."

U.S.authorities, however, are worried that the beheading of Flores will cause Mexican police to back off the murder investigation.

"We know that's a possibility, but we haven't heard any word that's going to happen," Tiffany Hartley said. "Obvioulsy we want David to be found and we don't want them to stop their search."

The widow said she gets search updates from Mexican authorities almost every day.

Zapata County, Texas Sheriff Sigifredo Gonzalez said he felt for Flores' family.

"They're going to now be without a father, without a husband when in fact all they were trying to do was search for the body of Mr. Hartley," Gonzalez said.

Flores' severed head was delivered Tuesday in a suitcase to a military post near the border with Texas. His wife had said he was working late on the Hartley case on Monday night and never came home.

Though many believe Flores' murder was the work of the drug cartels who control the waters where Hartley was killed, Mexican authorities have denied his death had anything to do with the murder investigation.

Hartley was shot Sept. 30 while on the Mexican side of Falcon Lake. Tiffany Hartley said the two were headed back to the American side of the lake on jet skis when they were chased by Mexican pirates. She told authorities that her husband was shot in the head and that she had to flee to shore to save her own life.

Hartley said she has been asked by Mexican authorities to travel to Mexico to give a statement, but she has declined to protect her own safety, instead filling out paperwork at the Mexican consulate.

Zeta Cartel Suspected in Decapitation of Top Investigator

Earlier this week, Mexican authorities named two suspects in Hartley's murder, two Zeta drug cartel members known as Pedro Saldiva Farrias, 27, and his brother Jose Manuel Saldiva Farrias, whose age was not given.

Juan Carlos Ballesteros, an investigator with the state prosecutor's office of the Mexican state of Tamaulipas, said both were said to be suspected members of the Zeta drug cartel from Nueva Ciudad Guerrero, near the abandoned town where David and Tiffany Hartley were sightseeing before they were ambushed.

The Hartley case highlights the ongoing turf war between Mexican drug cartels and the country's government, which has been waging a war against the growing drug violence. More than 29,000 people have died in drug violence since December 2006, with the Zeta cartel being blamed for the majority of the deaths.

Zeta is known for smuggling billions of dollars of cocaine and other drugs into the U.S. every year.

Mexican police had initially cast doubt on Tiffany Hartley's account of the incident, saying they could find neither her husband's body nor his Jet Ski.

But late last week, an eyewitness come forward, claiming that he saw Tiffany Hartley fleeing in panic to the American side of the lake.

"I saw the Jet Ski come around an island," the witness told "Good Morning America." "There was something wrong actually. The way I saw her come around, it looked like something terribly wrong happened. I mean, she was jittery, frantic. ... She was crying, sobbing."

Tiffany Hartley said she and her husband took Jet Skis to the Mexican side of the lake to take pictures of a small church when suddenly a band of Mexican pirates opened fire on them with assault rifles.

ABC News' Gina Sunseri contributed to this report.