Guterres is "deeply concerned" at reports of Capt. Rafael Acosta's death, and he urged a prompt, independent investigation, said U.N. spokesman Stephane Dujarric, adding that Guterres wants the government to bring anyone responsible for his death to justice.
Venezuela's chief prosecutor announced that two officers attached to a military intelligence agency had been arrested on homicide charges, though he made no reference to allegations of torture. Acosta died Saturday hours after his attorney Alonso Medina Roa says his bruised and bloody client was brought to court in a wheelchair, unable to stand from intense pain and struggling to speak, covered with cuts and with bloody fingernails and black eyes. He died a short time later after a judge ordered him transferred to a military hospital.
The government says that Acosta and five other members of the armed forces or judicial police planned to launch an operation on June 23 to kill Maduro and other top officials, including first lady Cilia Flores and socialist party boss Diosdado Cabello.
A manhunt is underway for eight other men suspected in the plot, officials say. Security forces arrested Acosta on June 21 on the outskirts of Caracas, Medina Roa said, adding that his client was healthy at the time.
Acosta's wife, Waleska de Acosta, has denied he planned to kill the president, while acknowledging that her husband opposed Maduro. She and her husband have two children aged 4 and 12, de Acosta said.
The death has drawn condemnation from the United States and several other nations as well as opposition leader Juan Guaidó, who renewed calls on the military to reject Maduro and join his movement to oust the socialist president.
Acosta's death follows a recent visit to Venezuela by Michelle Bachelet, the U.N.'s High Commissioner for Human Rights. Bachelet concluded her three-day trip calling on the government to free all those detained for "exercising their civil and political rights in a peaceful manner."
Bachelet expanded on Guterres' comments in her own statement Monday, saying she was "shocked" by allegations of torture, which may have been the cause of Acosta's death. "I remind the Venezuelan authorities that they are responsible for the life, and the physical and psychological integrity of all people deprived of their liberty," Bachelet said. "Structural measures should also be urgently adopted to prevent the recurrence of torture and other ill-treatment of people held in custody by the State."
Bachelet urged authorities to let attorneys and relatives visit six others arrested in the case to ensure they are treated with "humanity and dignity" and are "protected from torture and other forms of ill-treatment."
She said two members of the Directorate General of Military Counterintelligence, the agency that had Acosta in custody, have been charged on suspicion of first-degree murder. But she "regretted" that torture allegations didn't appear to be included in the charges.
Venezuela's chief prosecutor, Tarek William Saab, has said that Maduro ordered an exhaustive probe into the death. Saab later identified the two men arrested on homicide charges as Sgt. Estiben José Zárate and Lt. Antonio Ascanio, both members of the Bolivarian National Guard attached to the intelligence agency. The prosecutor said a preliminary investigation linked the men to the "regrettable" act.
Meanwhile, Minister of Foreign Affairs Jorge Arreaza said Venezuela is in talks with Germany to restore diplomatic relations nearly four months after Maduro declared German Ambassador Daniel Kieser a "persona non grata" for backing Guaidó and ejected him from the country.
Germany is among some 50 nations that back Guaidó as Venezuela's legitimate leader. Kieser was thrown out after meeting with Guaidó at the airport near Caracas in March with other foreign diplomats to prevent his arrest for leaving the country against a court order.
The German Embassy in Caracas did not immediately respond to an Associated Press request for comment.
Associated Press writer Edith M. Lederer at the United Nations contributed to this report.