Former Portuguese Prime Minister Jose Socrates remains in police detention while an investigating magistrate decides whether he should answer charges of corruption, money-laundering and tax fraud.

Socrates, who was Portugal's center-left Socialist prime minister from 2005 to 2011, was due in a Lisbon court Sunday for a closed-door interrogation.

Socrates, 57, was detained by police late Friday. The Attorney-General's office said he and three others are suspected of colluding in crimes.

The magistrate must decide whether there is enough evidence to bring charges against the suspects. One of them, Carlos Santos Silva, is reportedly a longtime friend of Socrates. His construction company flourished during Socrates' time in power.

Officials provided no further details, citing Portugal's judicial secrecy law which forbids the disclosure of details from ongoing investigations.