An Argentine former naval officer has been ordered to remain in custody throughout the investigation into alleged crimes during the 1976-83 period of military rule.
Astiz was told formally that immunity laws no longer apply
The judge, Sergio Torres, told Mr Astiz he no longer had the support of amnesty laws which had protected members of Argentina's former military junta from prosecution for alleged human rights abuses.
Mr Astiz is suspected by prosecutors of using a Navy school as a torture centre during military rule. He has also been convicted in absentia of the murders of two French nuns.
The 50-year-old - dubbed the "blond angel of death" - reportedly handed himself in on Tuesday after two judges ordered his arrest.
Mr Astiz is one of the first military figures to be detained since the repeal of immunity laws that for two decades protected members of the former junta against prosecution in August.
The former captain was being questioned on Wednesday, the Associated Press news agency reported.
But Mr Astiz's detention threatens to provoke tensions between representatives of those he is accused of ordering tortured and representatives of the two French nuns murdered in 1977.
Lawyers for the nuns' families demanded on Wednesday the French request for his extradition take precedence over legal proceedings in Buenos Aires, according to the Efe news agency.
But the families of those who "disappeared" during military rule have insisted he face justice at home.
Official figures say 9,000 people were kidnapped, tortured and killed in what became known as the "dirty war" - but most believe the real number to be closer to 30,000.
Argentine human rights groups have long demanded justice for those who tortured and murdered their left-wing opponents.
Files on the 1994 Jewish centre bombing have been reopened
The overturning of immunity laws is just part of a dramatic shake-up by Nestor Kirchner, who became Argentina's president in May.
Other steps by Mr Kirchner have included sacking eight out of 10 of the country's top police officers in a bid to stamp down on corruption, and the opening of sealed intelligence files on the deadly bombing of a Jewish centre in 1994.