SAN MIGUEL DE ALLENDE, MEXICO — Not long after Kathy and Jim Machir retired nine years ago, they left San Diego for a new home along the cobblestoned streets of this vibrant mountain town four hours northwest of Mexico City.

"We had been on vacation to San Miguel once and loved it," Jim Machir, now 72, said.

San Miguel de Allende is famous for its colonial architecture, bustling art scene, mild climate and low cost of living.

It has long been a magnet for American retirees, and more than 1,000 U.S. expats now call it home.

The Machirs sold their house in the U.S. and used the proceeds to begin building a new house in San Miguel de Allende.

But their retirement dream turned into a nightmare in December 2018 when they suddenly found themselves unable to pay their contractors.

Their story may send a chill down the spines of the more than 1 million other U.S. citizens, many of them retirees, who live in Mexico. The life savings they had entrusted to their local banker of more than six years had all but disappeared.

"I was speechless," said Kathy Machir, 67, recalling the moment she found out she had roughly 40 cents left in her account. "It just gives you a sense of ultimate betrayal, loss, horror…"

But the Machirs weren't the only ones.

Lani Van Petten is another Monex client who saw her money disappear in December of last year. She is the aunt of Marcela Zavala Taylor, the former banker accused of stealing money from several accounts in the San Miguel area. NBC News

NBC News spoke to nine American families who say Marcela Zavala Taylor, a former banker with Grupo Financiero Monex, had gained their trust only to disappear after they discovered money had gone missing from their personal accounts. These families, whose estimated losses total more than $7 million dollars, all say they were blindsided by what had happened.

NBC News has chosen not to disclose how much money each individual said they lost over concerns they expressed about their safety.

Jim Karger, 67, and his wife Kelly, 59, moved from Dallas, Texas to San Miguel to retire nearly 18 years ago. The Kargers said they began to rely on Zavala after the brick-and-mortar Monex branch in San Miguel had closed. She continued to represent Monex and handle the accounts of several clients in the area so they wouldn't have to travel.

"She was a very warm person," said Jim Karger, who, like the Machirs, alleges Taylor took advantage of his trust.

The Kargers and various other individuals say that Taylor's family is well-known in the area. Her father, Manuel Zavala, was once the town's mayor and her mother was an American. This, they say, contributed to their trust in Taylor. Taylor's parents are not accused of any wrongdoing.

"She wanted to know about your family. I knew about her family. We didn't consider Marcela Zavala our banker, we also considered her our friend," Karger said.

For a while, the Kargers said they had no trouble accessing their money as periodic statements would come in and money would be available through a runner.

And even last June, when the Kargers stopped receiving bank statements, they said Taylor told them that it was because Monex had been upgrading its computer system. At the time, they believed her. But by December, they finally realized something was wrong.

Kelly (left) and Jim Karger (right) moved to San Miguel de Allende from Dallas more than 18 years ago. They said they found out that nearly all the money they had put away through Monex was gone when Jim visited a Monex branch in the city of Quer?taro last December. NBC News

"She became harder to find, which was never the case," Jim Karger said. "Getting money was more difficult."

Karger's worst fears were confirmed when he visited the Monex branch in the nearby city of Querétaro.

"He said to me, 'It's all gone. All the money is gone,'" Kelly Karger said, recalling the phone conversation she had with her husband as she was out having lunch with a friend. "I just remember bending [over], just stopping in the street thinking, 'I'm going to be sick.'"

Devastated, Karger said she was eventually able to go through years of bank statements and determine that money was slowly siphoned out of their accounts.

However, it wasn't just regular clients who had allegedly lost money at the hands of Taylor, but family too.

Lani Van Petten, Marcela's U.S.-born aunt, said she was comfortable having a family member look after the money she had saved to buy a house and pay for her granddaughter's education.

"It worked smoothly because I never asked questions," Van Petten said. "I never got a statement. I thought, 'Oh well, you know, Marcela's on top of it, no problem.'"

That is, until December came around, and Van Petten needed to pay for her granddaughter's schooling.

"I had been telling her since mid-December, 'I've got to pay for Maria Jose's school, so, you know, send the money,'" she recalls telling Taylor.

But Marcela never answered, she said.

Van Petten then called Monex.

Kathy (left) and Jim Machir (right) moved to San Miguel de Allende from San Diego nearly nine years ago and had been building a house in the area when they found out all but $.40 was left in their accounts. NBC News

"They told me she no longer worked there," she said. "And they told me I had 1000 pesos in my account [about $50] and that it was all gone. I started to cry. That was the obvious thing to do."

In a statement to NBC News, Monex said 45 of the 50 complaints received from clients have been resolved and that the remaining five are still in negotiations.

"Monex reiterates that it is an institution that acts with strict adherence to national and international rules that all its businesses and operations fulfill according to law and are regulated and supervised by the corresponding authorities," Monex wrote in a translated statement.

Several of the people NBC News spoke to say getting their money back from Monex has been an uphill battle, especially when it comes to getting refunded for 100 percent of the amount they say was initially in their accounts. They also told NBC News they were upset with the way Monex responded to their concerns and believe the bank was involved in the alleged fraud. But many of them, including the Machirs and Van Petten, have since settled and said they were asked by Monex to pursue legal action against Taylor. They have signed confidentiality agreements that keep them from disclosing how much of their losses they were able to recover.

Monex also said it has initiated criminal legal action against the former banker and has strengthened its internal controls. However, Monex declined to say where or when it had filed any lawsuit against Taylor.

NBC News checked with Mexican legal authorities who said they have no record of any complaints related to the alleged fraud.

Taylor did not respond to several requests for comment.

Van Petten said that if she saw her niece, "I'd probably hold her hand and say, 'Marcela is this possible? Could you really have done all this?'" But Van Petten claims she has not been able to speak to Taylor or any of her family members.

The alleged victims say they don't want the hundreds of thousands of other Americans living in Mexico to go through the same thing they did and advise expats against placing all of their money in one spot.

"I think it comes down to doing your homework before you make any decision, and especially if you're going to do it in a foreign country," Kelly Karger said.

Kathy Machir agrees, adding, "Be alert. Be aware. Don't put all your eggs in one basket."

Didi Martinez reported from New York.