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An American missionary couple who were allegedly attacked by gunmen in a dangerous part of Mexico may have been targeted for their expensive pickup truck, because drug gangs covet the vehicles, police said Thursday.

Damage to the 2008 Chevrolet pickup truck that Sam Davis frantically drove against traffic across a border bridge Wednesday with his bleeding wife next to him suggests that another vehicle tried to run the couple off the road, Pharr police Chief Ruben Villescas said Thursday.

Nancy Davis was pronounced dead at a McAllen, Texas, hospital more than an hour after her husband reached the Pharr International Bridge border checkpoint. Her husband told investigators that that he and his wife were driving near the city of San Fernando, about 70 miles south of the Mexican border city of Reynosa, when gunmen in a pickup truck tried to stop them. When the Davises sped up, the gunmen fired, shooting Nancy Davis in the head.

Pharr said Nancy Davis was struck in the head by a bullet that shattered her vehicle's back window.

Authorities say the couple's heavy-duty truck is the kind prized by criminal organizations in Mexico, and similar to ones Pharr police say they can often single out as stolen before the vehicles are driven across the border.

"Driving that type of truck is an eye-catcher," said Pharr police Sgt. Ray Lara, who routinely patrols bridge traffic. "We figure maybe they don't bother the church people. But they want those trucks."

Lara described the truck as so conspicuous that he remembers seeing the couple drive it across the bridge several times. He believed the couple may have used the truck — which he estimated to be worth about $50,000 — to bring supplies into Mexico for their missionary work.

An autopsy on Davis, 59, was scheduled for Thursday. Pharr police chief Ruben Villescas said he planned to speak with Mexican investigators on Thursday, and that police interviewed Sam Davis into the night Wednesday.

Villescas wouldn't reveal where Sam Davis was Thursday or details about his church, citing the need to protect his family from criminal organizations who he said might now be after him.

"They were Christian warriors," Villescas said of the couple.

Wednesday's shooting echoes the disappearance and suspected slaying in September of an American tourist, David Hartley, while he was Jet-Skiing with his wife on a border lake. Tiffany Hartley said Mexican pirates attacked them and shot her husband while they were fleeing. His body was not recovered.

"I don't know them, but my heart breaks for them," Tiffany Hartley said of the Davises.

Concerns about the investigation into David Hartley's death led Texas Gov. Rick Perry to call for a stronger response from Mexican authorities. On Thursday, Perry's spokeswoman, Katherine Cesinger, said Nancy Davis' slaying underscores the need for greater border security.

"How many Americans are going to have to die for the federal government to pay attention and realize they need to secure the border," she said.

Villescas said Mexican authorities contacted by his department confirmed that Wednesday's shooting happened near San Fernando. The area is heavily controlled by the Zetas drug cartel and is one of Mexico's most dangerous. It is the same area where 72 Central and South American migrants were found slain in August, a massacre blamed on the Zetas.

A friend of the couple said the two spent 80 percent to 90 percent of their time in Mexico and had a home in the Mexican state of Nuevo Leon. The rest of the time they were in Texas and traveling across the U.S. raising funds for Gospel Proclaimers, the organization they founded.

"They've been working in Mexico for over 30 years," Merton Rundell III, the director of finance at Union Bible College in Indiana, told The Associated Press on Thursday. "It was mainly establishing churches — that was their main thrust.

"They loved the work they were doing in spite of the danger," Rundell said. "Yes, they were aware (of the dangers)."

Rundell, who has known Sam Davis since the two attended a Bible college in Florida together in the mid-70s, said he had spoken to Sam Davis since the shooting.

"It's still kind of hard to fathom," Rundell said. "I know it's happened but I guess the reality of it hasn't hit me yet."

Rundell described Nancy Davis as "a petite lady with a drive like you wouldn't believe. She lived life to the fullest. They were both totally given to it (missionary, teaching and evangelistic work)."

The Mexican Interior Ministry released a statement expressing condolences over Davis' death. It said Mexican authorities were investigating but provided no further details. Officials at the Tamaulipas state attorney general's office in Mexico could not be reached for comment Wednesday night.

U.S. Embassy spokesman Alexander Featherstone said the embassy was trying to contact Mexican authorities about the case. He could provide no other information.

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Associated Press writers Terry Wallace and Linda Stewart Ball in Dallas, and Alexandra Olson in Mexico City contributed to this report.