Concerns have heightened for the wellbeing of the “CITGO 6” – five U.S. citizens and one legal resident who have been detained by the Maduro regime in Venezuela for more than two years.

According to family members, the detainees suddenly disappeared from their various house arrest locations, presumably taken by the leading Venezuelan intelligence agency, known as the SEBIN.

“Last night we were suddenly informed that guards there had taken them,” Gabriela Zambrano-Hill, 25, whose father Alirio Zambrano and uncle Luis Zambrano, are among those behind bars, told Fox News on Thursday. “We were told they were getting some kind of psychological evaluation.”

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While some family members told Fox News their locations are unknown, Zambrano-Hill indicated that they were moved to the infamous Caracas building, El Helicoide, known for housing political prisoners.

The unexpected maneuver comes in the wake of President Trump hosting Juan Guaido, the opposition leader and U.S.-recognized president of Venezuela for his State of the Union address on Tuesday, in addition to rumblings of potential secondary sanctions placed on countries still supporting Maduro’s leadership.

In addition to the Zambrano brothers, both in their late fifties, former CITGO vice presidents Jorge Toledo, Tomeu Vadell, and Gustavo Cardenas and President Jose Pereira remain in custody.

“We are very worried about our father’s health and safety,” Cristina Vadell said of her 60-year-old father, Tomeu.

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Prior to their disappearance on Wednesday, she stressed they had had minimal contact with their dad and were not permitted regular phone calls.

“From a psychological standpoint, we can only imagine the trauma,” his other daughter Veronica Vadell said, pointing to the years her father spent in a prison basement without sunlight. “This is about human lives and human rights. This isn’t about politics.”

The American oil executives employed by the Houston-headquartered CITGO, a division of Venezuelan state oil company PDVSA, were initially seized by officials in late November 2017--after being lured to the Venezuelan capital of Caracas for a meeting--and slapped with corruption charges.

However, it is mostly assumed that the Americans are innocent and are being used as pawns between the U.S. and Venezuela, who have severed diplomatic ties and for years endured a withering relationship.

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Preliminary court hearings have been delayed some 15 times since their arrest, with little explanation. After more than two years behind bars, the men were moved to house arrest in December – raising hopes that they may be brought home.

Nonetheless, the latest developments have cast a shadow on the families who are hoping and praying for the safe return of their loved ones. Vadell’s family underscored that their dad had a pre-existing cardiovascular health condition and has not been able to receive adequate health checks. Moreover, he lost more than 60 pounds in the first year of incarceration – reportedly surviving on as little as 600 calories a day.

“We are just looking for cooperation; our father has never been a political guy,” Cristina said. “He was just doing his job; he came from a humble background. He is innocent. And besides that, by Venezuelan law, you cannot hold somebody for this long without a trial. We just want him home.”

The U.S. State Department did not immediately respond to a request for comment.

Vice President Mike Pence last year publicly pushed for the release of the men in April, after meeting with the distraught families.

“(They have) been forced to sleep with lights on and experience severe heat, lack of air circulation, constant and untreated sickness, overcrowding, malnutrition, and seeing less than 30 minutes of sunlight per month,” the families wrote in a joint letter last year.

Texas Senators John Cornyn and Ted Cruz, in addition to U.S. Sen. Bill Cassidy in Louisiana – where the main Citgo plant is located and where most of the men worked – have also echoed calls for their freedom.

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For the families in the U.S., each day is one of wincing unknowns, fears, and struggles. The saga unfurled the week of Thanksgiving 2017 when the men were summoned to that rare Caracas trip. Dennysse Vadell, who has been married to Tomeu for 34 years, vividly remembers begging her then-overworked husband not to go, as he was on vacation.

He promised to be back in just a few days – in time for Thanksgiving.

“There are no happy days, only bad days and better days,” she lamented. “This is time we are not getting back. We are very concerned for his health.”

The families of both Vadell and Zambrano confirmed that the executives’ salaries were cut off in early 2018, months after their arrest.

In a statement issued to Fox News on Thursday, a CITGO spokesperson said “CITGO continues to support the U.S. Government’s efforts to secure the release of our colleagues, who have now been detained for more than two years without trial.

“CITGO believes that the detention of these men violates their fundamental human rights, including the right to due process under the law. We pray for their safety, and for their families as they contend with all of the challenges presented by this lengthy separation from their loved ones,” the representative stated. “We continue to support the detainees’ families, and we are grateful for the efforts of this Administration and lawmakers on both sides of the aisle to bring these men home.”

Guaido did not respond to a request for comment, and it is unclear what efforts the opposition camp has been able to undergo to secure the release of the American detainees.

Given that the men held joint U.S. and Venezuelan citizenship, U.S. consular officials are said to have been repeatedly denied access to them.

However, Maduro has maintained that the men are “corrupt, thieving traitors” and initially chided U.S. demands to release them.

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And for Zambrano-Hill, the sounds of her five-month-old daughter’s soft cries mark a subtle reminder of the time the now-grandfather will never get back.

“It is such a terrible shame that these six innocent men have been embroiled in a political conflict that has nothing to do with them,” she added. “My dad is one of the hardest working and most honest people I know. This has left a huge hole in our families – not having the men here.”