This has been revealed in a story by Christina Ng writing on the ABC News website.
"They had opportunities to return, but these are their people. It would be like abandoning family. We don't abandon family," Karen Mosley, the secretary for Liberty Baptist Church in Lewisville, Texas, told www.ABCNews.com. "We lost a couple of great prayer warriors."
The story says that John and Wanda Casias were found dead at their home near Monterrey, Mexico, on Tuesday, January 31, 2012, by one of their 10 children. They had been living and working in Mexico for nearly 30 years and began their ministry there in 1983.
"It was the second time in a year that an American missionary was killed in Mexico. Nancy Davis, 59, was shot in the head in January 2011 when she and her husband were attacked at an illegal road block," said Ng.
"In the latest assault, Shawn Casias had come to pick up a trailer from his parents' home and when he went inside, he found his mother Wanda Casias, 67, lying on the floor with an electrical cord around her neck and a head wound, according to the Associated Press. His father John Casias, 76, was found in the storage room of a building on the property with an electrical cord around his neck."
A safe, several electronics and the couple's car were missing from the home. All of the surveillance cameras had been disabled, said ABC News.
"They had all kinds of updated security measures, which makes us all believe it was somebody he knew or he wouldn't have let them in," Mosley said. She said they lived on a walled property and had a gate.
Mosley said church members used to go on missions every other year to the area where the Casias couple lived, until about four years ago when things became too dangerous for them to travel there. Fighting between drug cartels over the past few years made Monterrey an extremely dangerous place.
"John doesn't allow visitors to come to his place anymore. It's that bad," Mosley said. "There are no more policemen, they're all dead. The last mayor was murdered. All they have there are soldiers."
The couple had discussed what they wanted, should anything ever happen to them, with members of the church, Mosley said. The couple said that if they were to be kidnapped, they did not want anyone to pay ransom for them.
"They would not allow it," Mosley said. "They knew it was dangerous. They were just cautious and they made a pact."
The couple maintained a website, www.casias.org, with details and photos about their missionary work.
"John was a character. He was one funny person, but he took his love of his Lord very seriously. He was hard working, loving, caring and would give you whatever you need, if he had it," Mosley said. "Wanda was a very sweet lady, always got a hug and a smile. She would ask, 'What can I pray for you for?' She was always praying."
The attack on the Casias comes year after Nancy Davis and her husband Sam were ambushed.
"The couple, also from Texas, were driving along a highway 70 miles south of the border town of Reynosa, Mexico, when they approached an illegal roadblock. Rather than stop, they continued driving and were chased by several gunmen in a black pick-up truck," added Ng in her story.
"The pursuing gunmen fired at Davis' truck, and a bullet hit Nancy Davis in the head. Sam Davis sped away to a border checkpoint and his wife was rushed to a hospital, but she did not survive."