A U.S. diplomat visited jailed American oil executives this week in Venezuela in a meeting that was not previously announced, the State Department said.

“We can confirm that the Special Presidential Envoy for Hostage Affairs Roger Carstens traveled to Caracas for discussions about the welfare and safety of U.S. nationals in Venezuela,” the State Department said in a statement to The Hill. 

Four people familiar with the visit told The Associated Press that Carstens went to Caracas for four days and spoke to six jailed American oil executives from Citgo. 

Tomeu Vadell, Jose Luis Zambrano, Alirio Zambrano, Jorge Toledo, Gustavo Cardenas and José Pereira were jailed in Venezuela in 2017 on embezzlement charges in a trial that had many irregularities and is considered illegitimate by the the U.S.

In the 90-minute meeting, Carsten told the oil executives he spoke to representatives for Venezuelan President Nicolas Maduro about their case to try to obtain their release, one source told the AP. 

The men were sentenced from eight to 13 years in prison, but were granted house arrest. However, they were thrown back into prison after a Maduro ally was extradited to the U.S. this year. 

After returning to prison, one of the oil executives suffered a heart attack. 

The meeting with Carsten comes after a Venezuelan judge granted a rare appeal for the six oil executives for their corruption case in November.

“We continue to advocate for the immediate and unconditional release of all wrongfully detained U.S. nationals in Venezuela at every opportunity,” the department said.

Updated at 5:43 p.m.