Portuguese police have arrested former Socialist prime minister José Sócrates and three other people in an investigation of suspected tax fraud, corruption and money-laundering, the prosecutor general’s office said on Saturday.

The detention, the first involving a former premier in Portugal under democracy, follows arrests of other high-ranking officials or prominent people in separate inquiries in the past few months as prosecutors intensify a fight against corruption in a country notorious for its slow justice system.

Sócrates was brought before a criminal judge for questioning on Saturday afternoon after his apartment was searched. The prosecutor’s office said the questioning and search were part of an investigation of suspicious banking operations and money transfers, but would not reveal more details.

It was not clear if the investigation was linked to Sócrates’ time as prime minister between 2005 and 2011.

Police arrested Sócrates, 57, at Lisbon airport late on Friday as he arrived from Paris. He spent the night in a police station, according to a police official. The other three arrested individuals are linked to Sócrates.

He resigned as prime minister in the middle of his second four-year term in 2011 as an escalating debt crisis forced him to request an international bailout, which imposed painful austerity on Portugal.

His centre-left Socialist party leads in opinion polls ahead of next year’s general election.

Socialist leader Antonio Costa told party militants in a message that “we are all certainly shocked” with the detention, but added that “feelings of solidarity and personal friendship” should not influence the party’s policy or interfere in any way with the independence of the investigation.

“The party has to focus on mobilising Portugal in affirming an alternative to this government and its policy,” he wrote.

A snap election in 2011 brought to power the current centre-right ruling coalition, which introduced the unpopular austerity measures such as tax hikes and cuts in wages and pensions.

After stepping down, Sócrates left for Paris, where he attended a university course. He returned in 2013 to become a regular commentator on RTP state television.

During his premiership, he weathered several investigations, including allegations that he misused his position as environment minister in 2002 to allow the construction of a shopping mall. He denied wrongdoing and faced no formal charges.

Prosecutors have detained and are investigating several prominent people in separate corruption and fraud cases.

Last week, the head of Portugal’s immigration service, Manuel Palos, was arrested along with several other officials on suspicions of corruption linked to the issuing of so-called “golden visas” to wealthy foreign investors. The inquiry also forced interior minister Miguel Macedo to resign.

*Source: Reuters