Rights groups have expressed concern for Venezuelan activist Rocío San Miguel, who has not been heard from since she was arrested on Friday.
Ms San Miguel is a vocal critic of President Nicolás Maduro's government.
She has been accused by the prosecutor general, who is a close ally of Mr Maduro, of involvement in an alleged plot to kill the president.
She and five of her relatives, who have also been detained, are being held at undisclosed locations.
A member of her legal team, Juan González, said five members of her family were unaccounted for and the authorities were not responding to his requests for information.
On Monday, Prosecutor-General Tarek William Saab wrote on X, formerly known as Twitter, that Ms San Miguel had appeared before a tribunal, where she was charged with "treason, conspiracy, terrorism" among other things.
He had earlier accused her of involvement in an alleged plot to kill the president. He said that the aim of the "conspiracy" had been to kill President Maduro and other high-ranking officials, as well as attacking several military units in the city of San Cristóbal.
It is not clear why her daughter, her ex-husband and three other members of her family have been detained.
Mr González said "anyone who is emotionally attached" to Ms San Miguel was being arrested, adding: "I think this is a dire warning for those of us who have resolved to defend the human rights of people who are imprisoned or persecuted for political reasons, and also for those who document human rights violations in Venezuela."
The Office of the United Nations High Commissioner for Human Rights expressed "deep concern".
"Her whereabouts remain unknown, potentially qualifying her detention as an enforced disappearance," it wrote on X. "We urge her immediate release & respect for her right to legal defence."
Fifty-seven-year-old Rocío San Miguel is an expert on defence issues who leads the Control Ciudadano NGO, which advocates civilian oversight of Venezuela's armed forces.
Rights activists first sounded the alarm on Friday, when Ms San Miguel was detained by intelligence agents at Simón Bolívar international airport, near the capital, Caracas.
Her arrest came just weeks after 36 government critics were rounded up.
They, too, were accused of having links to alleged plots to kill Mr Maduro.
The wave of detentions comes as the government is preparing to announce the date of the presidential election, which is due to be held later this year.
The government had agreed in talks held in Barbados with opposition representatives to lay the groundwork for the election to be held freely and fairly.
But rights groups say that rather than progress, there have been setbacks since the agreement.
Most notably, a ban which prevents the main opposition candidate, María Corina Machado, from running for office was upheld by the Supreme Court.
Members of her Vente Venezuela party are among those who have been accused of being part of the alleged plot against Mr Maduro.
Ms Machado has in the past denounced the arrests as part of a campaign aimed at intimidating her and suppressing any opposition to Mr Maduro, who has been in power since 2013.