Story highlights Hartley's widow calls Monday's arrest an "out-of-the-blue shock"

Salvador Martinez Escobedo is suspected in the Falcon Lake killing of David Hartley

Authorities have not said why or how they believe he was involved in the slaying

A Texas sheriff says the suspected Zetas leader was not on his list of suspects

For more than two years, details about the investigation into who killed David Hartley have been as murky as the waters where his body disappeared.

His grieving widow told police that attackers shot him in the head on a lake that straddles the U.S.-Mexico border. She said she was forced to flee and leave his body behind.

Some blamed her for his death. Then the severed head of the case's lead investigator was delivered in a suitcase to a Mexican military post.

The trail seemed to go cold until Monday, when Mexican authorities said they had arrested a leader of the ruthless Zetas drug cartel and linked him to the Falcon Lake killing.

Salvador Alfonso Martinez Escobedo also is a suspect connected to the slaying of the Mexican state police investigator who was heading the investigation into Hartley's killing until he turned up dead himself nine days later, the Mexican navy said in a written statement.

Hartley's widow, Tiffany Hartley, told reporters in Colorado that the news was "a complete, out-of-the-blue shock." She said she had heard no news about her husband's killing for 10 months, and she and her family were trying to learn more "from anyone."

"We've got calls out to all these different agencies, and we haven't heard back from any of them," she said.

Authorities accuse Martinez, known as "The Squirrel," of being a regional leader of the Zetas in three northern Mexican states and heading up a brutal battle with the rival Gulf cartel. They had offered a reward of more than $1 million for information leading to his capture.

He was picked up in the Mexican border city of Nuevo Laredo on Saturday night and presented to the media on Monday. But one Texas sheriff says the 31-year-old alleged cartel leader wasn't on a list of five suspect names in the Hartley case that he's handed over to the FBI.

"Based on the information I have, he may have been the one responsible for that area, but not the one responsible for the actual killing," said Sigifredo Gonzalez, sheriff of Zapata County, Texas.

The navy didn't explain why or how authorities believe Martinez is connected to the case.

But his arrest appears to be the first that officials have publicly tied to Hartley's killing.

"If this is, in fact, true that he was involved," Gonzalez said, "it's a welcome sign that the Mexican government is trying to solve the killing of yet another American citizen on Mexican soil."

Widow: Cartels 'are killing anyone who gets in their way'

After the shooting she reported on September 30, 2010, Tiffany Hartley told a dramatic story of the attack to investigators. For months, she told the story to lawmakers and members of the media, hoping, she said, to speak out for others caught in the drug-related violence that has killed more than 47,500 across Mexico since December 2006.

"The men who murdered David are right across the river. They aren't in Afghanistan, they aren't in Iraq. They're in our own backyard," she said at a congressional field hearing in Brownsville, Texas, last year. "The cartel members are taking over Mexico, and they are killing anyone who gets in their way."

Tiffany Hartley has said that she and her 30-year-old husband were ambushed by assailants while riding Sea-Doo personal watercraft on the lake.

It was supposed to be leisurely adventure, she said. They were visiting the ruins of a half-submerged church.

They'd been snapping photos when three boats carrying armed men approached, she said. Bullets whizzed by as the couple took off on their Sea-Doos. One of them hit David Hartley in the head.

She said she was unable to haul his body onto her watercraft before being forced to flee.

Some raised doubts about her account. In a 2010 interview with CNN, she said she was hurt by the suspicion, but no, there was no drug deal gone bad. She said she was no merry widow, as one border town official suggested. Her heart is broken, she said.

"You know, the only people who know what happened that day are the people on the boat, God and myself. The only people who I cared about believing me are my family and his family," she said. "Everyone else can doubt me as much as they want."

Seven months later, a member of Mexico's navy and 12 suspected members of the Zetas were killed in a shootout on a Falcon Lake island. Drug traffickers were using the island for storing marijuana to be transported by boat to the United States, the navy said in a statement at the time. After the shootout, the navy said it seized guns, ammunition and bullet-proof vests.

Hartley's remains have never been recovered, and Tiffany Hartley said Monday that she hopes his body is found as a result of the arrest. She said her confidence in authorities has been hard to come by two years later, but, "At the same time, we're hopeful."

Suspect's alleged ties to high-profile crimes

Some Mexican reports Monday about Martinez's capture mentioned his alleged ties to Hartley's killing. But his arrest drew national attention for his alleged connection to some of the region's most high-profile crimes, including the 2010 massacre of 72 migrants in Tamaulipas , in addition to other slayings, mass graves and large-scale prison breaks in northern Mexico.

Martinez is accused of leading the Zetas in the Mexican states of Tamaulipas, Nuevo Leon and Coahuila, authorities said. Mexican officials also said Monday that he is suspected in "the execution of more than 50 people by his own hands in different parts of the country."

Shortly after Hartley's death was reported, authorities surmised that the couple had stumbled into the middle of a drug transaction. Gonzalez, the Zapata County sheriff, has said there were reports of fishermen on the lake being warned away from the Mexican side as long as a year before Hartley's shooting.

Word has eked out about possible suspects in the case. But information on the investigation has seemed scarce.

Last year, Tiffany Hartley sued the State Department, the Justice Department and the FBI in an attempt to get answers and find out why no one had been brought to justice in her husband's killing. All three lawsuits were dismissed this year after settlements were reached, according to court documents.

But the case has remained unsolved.

"It didn't happen in the United States," Gonzalez told CNN last year, adding that Mexican authorities have "somewhat of a zero solvency rate, and a zero conviction rate."

"So unfortunately," Gonzalez said at the time, "this case may remain open forever, even though the information and the evidence may be there."