Updated

A jailed councilman in Venezuela arrested on suspicion of involvement in a failed assassination attempt on President Nicolas Maduro has died of suicide, the government said Monday.

The official suicide claim drew swift denial from opposition leaders close to Fernando Alban, and a few dozen of his supporters gathered outside the headquarters of Venezuela's intelligence service yelling "Maduro killer!" contending that he had been murdered.

While Venezuelans last year watched as dozens of youth were killed in violent street battles with security forces, the death of activists while in the government's custody is a fate more associated with the far deadlier dictatorships that dominated much of South America in the 1970s.

Alban, 56, was taken into custody Friday at Caracas' international airport upon arriving from New York, according to his lawyer. He was in the U.S. accompanying other members of his First Justice party for meetings with foreign dignitaries attending the United Nations General Assembly.

Opposition leader Julio Borges, who led the delegation to the U.N., said Alban's wife told him that her husband had been under intense pressure to testify against him in the ongoing probe into the alleged plot in early August to kill Maduro using two drones loaded with explosives.

More than two dozen people have been jailed on suspicion of involvement in the alleged plot, which Maduro claims was orchestrated by Borges with the support of Colombia and the U.S.

"There's no doubt this was an assassination," Borges said in a video from exile in neighboring Colombia. "The only thing left for this government is torture, violence and destruction."

Chief prosecutor Tarek William Saab ordered an investigation into the circumstances surrounding Alban's death, which he classified as a suicide.

In brief comments on state TV he said Alban was in the waiting room of the Caracas headquarters of Venezuela's intelligence police waiting to be transferred to a courthouse when he asked to use the bathroom. He then threw himself from the 10th floor of the building,

Republican Sen. Bob Corker, the outgoing chairman of the Senate Foreign Relations Committee, who is in Venezuela for meetings with Maduro and his opponents, called Alban's death while in the government's custody "disturbing."

"The government has a responsibility to ensure all understand how that could have happened," he said in a message posted on Twitter.

Also expressing concern was Luis Almagro, the head of the Organization of American States and a sharp critic of Venezuela's socialist government, who called Alban's death "the direct responsibility of a torturing and homicidal regime."

"This criminal dictatorship should leave Venezuela now," he tweeted.

As night fell on Caracas, and rumors swelled on social media that the government was planning to cremate the body to hide any signs of torture, family members gathered outside the morgue to demand Alban's body be handed over.

Borges, who said Alban was a personal friend, said the councilman who represented a district in the Caracas area was a family man and devout Catholic who would never kill himself.

"Alban is a very Christian person, with deep spiritual convictions that go contrary to a decision to take one's life," said lawyer Joel Garcia, who has represented Alban.

He said he met with Alban the night before in the tribunal and his client had seemed calm. Garcia said authorities cannot determine that a death is a suicide without an investigation and he would ask to be present at the autopsy.