An investigative judge in Portugal questioned former Prime Minister José Sócrates on Sunday to determine whether to charge him with tax fraud, corruption and money-laundering in a probe that could hurt his Socialist Party as it seeks to return to office in next year's elections.

Mr. Sócrates, who was the country's prime minister from 2005 to 2011, was detained at Lisbon's airport late Friday when he arrived from Paris and has been held since in a police jail.

The prosecutor general's office said it had initiated the inquiry after a Portuguese bank reported suspicious banking operations and money transfers. The announcement didn't specify the time frame for the allegations or give other details, and no charges have been filed against Mr. Sócrates.

The former Portuguese leader's defense lawyer, João Araújo, said he wouldn't comment on the substance of the inquiry until Judge Carlos Alexandre finished the closed hearing, which was set to continue on Monday. The lawyer said only that his client had been "in a great and strong state of mind" since his detention and during Sunday's four-hour hearing.

The 57-year-old Mr. Sócrates, a civil engineer by training, styled himself as a modernizer during his tenure as prime minister. While he was in office, his government loosened restrictions on abortion and introduced gay marriage in the mostly Roman Catholic country and helped make Portugal a leader in Europe's drive to adopt renewable energy.

His popularity, though, sank after overspending and soaring government debt forced Portugal to take an international bailout in 2011. He resigned that year, in the middle of a second four-year term, after Parliament rejected an austerity program put forward by his minority government, triggering an early election won by the center-right coalition that now governs Portugal.

It wasn't clear whether the investigation covers Mr. Sócrates's tenure as prime minister. During that time, he faced several judicial inquiries, including questions over whether he had misused his position as environment minister in 2002 to allow the construction of a shopping mall. He denied wrongdoing and the case was shelved in 2010, with no charges filed.

After stepping down as prime minister, Mr. Sócrates moved to Paris, where he studied at a university and worked for a Swiss-based company as an international business consultant. He returned to Portugal last year to start a career as a commentator on public television.

In an interview in July with Portuguese state television, Mr. Sócrates denied any involvement in an unrelated money-laundering probe. He said he couldn't hide money because he didn't have much. He said he had taken out a loan to live in Paris while studying.

Mr. Sócrates's detention is a setback for the Socialist Party and its newly chosen leader, popular Lisbon Mayor Antonio Costa, a close ally who had served as a minister in his government.

In a message to party members before just his election to lead the party, Mr. Costa sought to distance himself from the former prime minister, saying "it isn't for the party to evaluate" the case. "Whoever has committed an illegal act, that act will be evaluated and judged by the justice system," he said.

Recent opinion surveys of voters have put the Socialists in the lead over the parties in Prime Minister Pedro Passos Coelho's center-right coalition. Mr. Passos Coelho's government has lost popularity as it carried out austerity policies demanded by the country's bailout lenders, including tax increases and cuts in wages and pensions.

The government has also been hurt by scandals. Interior Minister Miguel Macedo resigned last week following the detentions of several senior government officials in an investigation into money laundering and influence-peddling in the awarding of so-called "golden visas."

The program allows foreigners who buy property worth EUR500,000 ($620,000) or more and keep it for at least five years to receive residency rights in Portugal and visa-free travel throughout the European Union's Schengen zone.

Patricia Kowsmann in Lisbon contributed to this article.

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