Argentine newspaper Clarin reported on Wednesday the existence of a secret third entrance to the flat – publishing photographs of a small metallic door which gave access to the apartment.

The third door linked Mr Nisman’s flat to a neighbouring apartment, which was rented by a foreign national – not thought to be Iranian. Clarin said that in the narrow corridor between the two doors, blocked by an air conditioning unit, police had found a fingerprint and a footprint.

The 51-year-old had on January 14 spoken of his investigation into the 1994 AMIA bombing – an attack on a Jewish centre in the heart of Buenos Aires, which killed 85 people and injured 300.

Iran has long been suspected of orchestrating the terrorist plot, and Mr Nisman said that Cristina Kirchner, the Argentine president, had agreed with Iran to protect those involved, in return for favourable deals on oil. Iran has always denied responsibility, and Ms Kirchner has dismissed the allegations as nonsense, and a distraction from the task of finding out what really happened.