(CNN) The phone connection between a Venezuelan prison cell and Houston, Texas, crackles and fades.
Jorge Toledo sounds tired. He's been held for four years, one of six former executives of Venezuela's US-based oil refining company CITGO convicted of corruption.
The sounds of prison life echo as he greets his wife, Carmen.
"Hola mi vida. I wanted to talk to you a little bit," he says.
These calls are his only link to the outside world and his family back in the United States.
Besides Toledo, the "CITGO 6" include Venezuelan-born American citizens Gustavo Cárdenas, José Luis Zambrano, Tomeu Vadell and Alirio José Zambran, as well as permanent US resident José Ángel Pereira.
They have become pawns in the long-running confrontation between the United States and the Venezuelan regime of embattled President Nicolas Maduro, a CNN investigation has found. US government and private efforts to win their release have come to nothing, and they now face a fifth year incarcerated.
Toledo's health has suffered during four years of deprivation, both his family and lawyers say.
"When he left here he was 176 pounds, an active marathon runner and an athlete, and he went all the way down to about 120 pounds in the first 10 months. At that point we were allowed to start bringing them food," says his son Carlos Añez.
"The first time I saw him on camera, I broke down in tears," Añez says. "It's been very difficult for everybody. I can't even begin to comprehend what he's gone through."
The family's nightmare began just before Thanksgiving in 2017.
Toledo was relaxing at home in Houston on a Saturday evening, when he received an urgent message summoning him to a meeting in Caracas. A private jet owned by CITGO would take him and his colleagues to Venezuela the following day.
The group of men that would become known as the CITGO Six flew to Caracas the next day.
Toledo texted Carmen on the third day of their trip. "Right now, we are going to PDVSA [the Venezuelan state oil company] because they called us to have a new meeting, and then we will go back to the US."
A few hours later, all six were in custody. And in a nationally televised news conference a day later, Maduro suggested their fate was already decided.
"These people were born in Venezuela, they are Venezuelan, and will be judged as corrupt, thieves and traitors to the homeland," he said.
"My blood boils when I think about them, they are cheaters and the full weight of the law must fall on them."
Gen. Manuel Christopher Figuera was a member of Maduro's inner circle when the CITGO 6 were arrested.
He had a lengthy career with Venezuelan security services, having served 12 years under Maduro's predecessor Hugo Chavez, before becoming deputy director of the military counterintelligence (known as DGCIM) in 2007, head of Strategic Center for Security and Protection of the Homeland, and then head of the nation's intelligence agency, SEBIN, in 2018.
The US Treasury Department sanctioned him in February 2019 for being part of Maduro's intelligence apparatus, which it alleged "has suppressed democracy, including through torture." But he later broke ranks with Maduro and supported opposition leader Juan Guaido in a failed attempted to overthrow the government.
Christopher Figuera denies the allegations he committed abuses but says he feels "guilty" for the role he played within the regime. Now in the US, he says the order for the arrest of the CITGO 6 came straight from Maduro.
"The general in charge of the investigation, Ramon Balza Liotta, consulted me in a few matters in order not to commit any excesses," Christopher Figuera told CNN.
"I asked him several times, where is the arrest warrant? And he told me there was no arrest warrant, it was a direct order given by Maduro."
But why would Maduro want to imprison six executives of the profitable US subsidiary of the country's state oil producer and arguably its most important company, PDVSA, and risk further poisoning relations with the US?
The military officer who arrested them, Gen. Balza Liotta, said Venezuelan authorities had received information about alleged embezzlement committed by the oil executives from intelligence sources in the United States, according to court transcripts reviewed by CNN. No evidence supporting his claim was ever presented in trial, those transcripts show and defense lawyers also say.
The government also claimed the six had been involved in an illicit project to refinance CITGO's debt. Maduro himself spoke of "an illegal, unconstitutional and criminal way to make commercial deals without the due authorization from the highest authorities in Venezuela."
But documents obtained by CNN show that the board of CITGO's parent company explicitly authorized negotiations. Venezuela's then-Foreign Minister and current Vice President Delcy Rodriguez was among the documents' signatories.
All the CITGO 6 pleaded not guilty to the accusations in the closed-door trial that finally began in August 2020, nearly three years after they were detained. But on November 26, 2020, five of the six were sentenced to eight years and ten months. Jorge Pereira received 13 years.
Former regime insiders say Maduro needed scapegoats as Venezuela's economy collapsed in a tide of corruption and mismanagement of PDVSA and the oil industry. The arrests of the CITGO 6, among other oil executives, provided the regime with such targets as anti-government protests escalated in 2017.
In the largest of those protests, named 'the mother of all marches,' on April 19, 2017 at least three people were killed.
Maduro was starting to feel the heat, says Christopher Figuera.
"There was a lot of pressure on Maduro in 2017," he told CNN.
Americans made an obvious target for the regime, especially after then-US President Donald Trump escalated sanctions against PDVSA in 2017. From that moment on, CITGO was unable to repatriate much of the proceeds from sales in the US market, cutting Venezuela off from a critical source of revenue.
Rafael Ramirez, a former PDVSA head who also served as Venezuela's Foreign Minister and UN envoy, believes Maduro has always suffered from not having the authority of his predecessor, Chavez, despite being his hand-picked political heir.
Maduro gutted PDVSA of people associated with Chavez, using allegations of corruption, points out Ramirez. "[Maduro] fired ministers, some of the great figures of 'Chavismo,' and carried on attacking them, accusing them, forcing them into exile. And the shock policy was accusing them of corruption," he says.
At the time, Venezuela's Prosecutor General William Saab said: "They're saying this is all part of an internal struggle. What internal struggle? This is corruption, corruption of the rottenest kind."
CNN has reached out to the Venezuelan government for comment but has not heard back.
Since 2017, 15 CITGO or PDVSA employees have been arrested on corruption charges without any real investigation, according to Venezuelan NGO Foro Penal. It was an extensive purge -- reaching the highest levels.
Nelson Martinez, who ran PDVSA, was arrested nine days after the CITGO 6 and died in prison a year later. Former Oil Minister Eulogio del Pino was arrested alongside Martinez. Del Pino had been replaced as oil minister days before his arrest by Gen. Manuel Quevedo, who had no prior experience in the oil sector.
The CITGO 6 were victims of the same purge, says Ramirez, who resigned from his government post one week after their arrest and fled into exile. "The order to detain them and the way it happened is an instruction by Maduro to sow terror, to generate fear," he says.
"Maduro's government is sustained by a balance of internal forces," Ramirez explains. "In the face of Maduro's failures in the management of PDVSA, the military exerted pressure to take over the industry."
For four years, the CITGO 6 have been used as bargaining chips by the Venezuelan government, a CNN investigation has found.
They were released into house arrest in December 2019, only to be sent back to prison two months later, after Trump welcomed Guaido, the opposition leader, to the White House.
They were moved to house arrest a second time, in May 2021, in what then-Venezuelan Foreign Minister Jorge Arreaza called a goodwill "gesture" towards the new administration of US President Joe Biden.
Five months after that, they were sent back to prison again, after the extradition of Colombian businessman and alleged Maduro financier, Alex Saab, to the United States.
Former US Ambassador to the United Nations Bill Richardson has been navigating this political minefield to secure the freedom of the CITGO 6, acting as what he describes as an informal intermediary between Caracas and Washington.
"It seems they have been used as bargaining chips," Richardson says, after spending more than a year negotiating with the Maduro government. "They deserve to come home."
"From what I know of these men, they're good people," he adds.
Richardson believes there's a small window of opportunity for the Biden administration to try to negotiate with the Venezuelan president. The US and Venezuela have been at odds since the Obama administration, when sanctions against some within the regime were first introduced. It was the last time Washington had an ambassador physically in Caracas. The relationship deteriorated further when Trump introduced sanctions against Maduro and some of his closest allies.
"Maduro was very unhappy with the Trump administration policy of sanctions so the atmosphere in my first visit was not good, even though I've known Maduro for some time," Richardson says. "They don't want any further sanctions, they want sanctions lifted."
The lack of any official diplomatic relations between the US and the Maduro regime makes progress more difficult.
"What I've tried to convince the Maduro government (of) is that soft diplomacy, personal diplomacy, humanitarian acts, pave the way for political progress," Richardson says.
"The relationship has been so poor, in the last four to eight years that I'm the one that's talking to the men as well as the US government," he adds. "The US government doesn't talk to them."
The families of some of the CITGO 6 feel that more can be done, and that the Biden administration has let them down. The imprisoned executives themselves have lamented the stalled negotiations, according to their families.
"There's been a lot of missed opportunities and we really need the help of top leadership to understand what's being done," says Cristina Vadell, daughter of Tomeu Vadell. "It's been an ambiguous policy with the administration."
"This tug of war is just too much," says her sister Veronica.
Añez, Toledo's son, is also exasperated with the lack of progress.
"They've had plenty of time to review their foreign policy, make changes, and it seems like foreign policy and politics are more important than Americans to the administration so far," he says.
"Until they decide to put those differences aside and talk to the Venezuelan Government my dad will continue to remain in prison."
The US State Department has said that it continues to press for their release.
"We continue to seek the unconditional release of Jorge Toledo, Gustavo Cardenas, José Pereira, Tomeu Vadell, José Luis Zambrano, and Alirio Zambrano from prison in Venezuela," the State Department said. "Nicolás Maduro has the power to release them -- and all US nationals who are wrongfully detained in Venezuela. We urge him to allow them to return to their families in the United States."
Jorge Toledo's once successful and comfortable life has now become a struggle to survive.
He and the other executives are being held in crowded cells, with no windows, their lawyers say. The Venezuelan prison authorities provide almost nothing, and it's the families of the CITGO 6 who have to deliver food, water, medicine and any other basic items they might need.
On his last call, Toledo left a message.
"One thing I wanted to ask is that, for the weekend, I'm going to need a bar of soap."