Fernando Albán was detained over what officials say was a drone assassination attempt on President Nicolás Maduro.
In a Twitter post on Monday, Reverol said that while in the waiting room on the 10th floor of the headquarters of state intelligence agency SEBIN, Alban had jumped from the window to his death.
An opposition member alleged to have taken part in a failed exploding drone "attack" on President Nicolas Maduro killed himself in pretrial detention at intelligence headquarters, Venezuela's top prosecutor said on Monday.
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 Maduro government has "an obligation to ensure (Alban's) safety, personal integrity and dignity", a spokeswoman for the United Nations human rights office in Geneva, Ravina Shamdasani, told reporters.
"We are concerned about news of his death". The European Union said it expected a "thorough and independent investigation".
Maduro has said the alleged August attack amounted to an assassination bid involving explosive-laden drones and his government and has accused neighbor Colombia of shielding the culprits.
According to the opposition party's coordinator Julio Borges, Alban's lifeless body was then thrown from the SEBIN's headquarters, located on Caracas' Plaza Venezuela. It did not provide evidence for its charge or say how it believed he was killed.
The party said that Mr Alban's arrest was because of statements he made at recent United Nations meetings in NY denouncing human-rights violations in Venezuela.
People stand next to the coffin of the opposition lawmaker Fernando Alban during a ceremony at the National Assembly in Caracas, Venezuela October 9, 2018. On Monday, Reverol said Alban had also been investigated for "destabilizing activities directed from overseas". Mr Corker was in Caracas on Monday to meet government officials and the opposition to assess Venezuela's economic crisis. "We understand there are conflicting reports on what exactly happened". The UN rights office also raised the alarm on Tuesday over the state of 59 Colombians held in Venezuela for more than two years without being charged.
The government has denied that it is detaining people as political prisoners and has said that they were fairly jailed.
"We will be looking at all issues that are related to the human rights situation in Venezuela", Shamdasani said.
In November of a year ago, a Venezuelan judge ruled they should be unconditionally released.
Venezuelan prisons are overcrowded and conditions are "horrible", with inmates deprived of food and drinking water, Shamdasani said on Tuesday.