Portugal Prime Minister Antonio Costa resigned unexpectedly on Tuesday. His resignation came hours after the police raided government buildings as part of an inquiry into corruption and “influence peddling” and issued an arrest warrant for Costa’s chief of staff, The New York Times reported.

Costa had been in power since 2015.

In his televised remarks, he said that he had been “surprised” to learn that he would be the subject of criminal proceedings and that “no illicit act weighs on my conscience.”

“However, I believe that the dignity of the office of the prime minister is not compatible with any suspicion about your integrity or your good conduct and even less with the practical suspicion of any criminal act. Therefore, in these circumstances, obviously, I presented my resignation,” he added.

A judge authorised the police to search 37 locations, including the office of Costa’s chief of staff, the Ministry of Environment, the Ministry of Infrastructure, a City Council office in the town of Sines and several private homes, according to a statement from the prosecutor general’s office, as per The New York Times.

The investigation relates to lithium exploration concessions in northern Portugal and a hydrogen-energy production plant and data centre in Sines, on the country’s southern coast, the statement said.

Though the statement did not name Costa, arrest warrants were issued for the head of the prime minister’s office–identified by local media as Vitor Escaria–along with the mayor of Sines and three other individuals. Portugal’s minister of infrastructure and the head of Portugal’s Environmental Agency were also named as suspects in the statement.

The prosecutor’s office said that the investigation showed that the suspects had invoked Costa’s name and authority “to unblock procedures” in relation to the exploration concessions.

Portugal has significant reserves of lithium, an essential ingredient in electric car batteries and renewable energy.

The leader of the Socialist Party, Costa, took office in 2015 when he lost an election but ended up becoming prime minister anyway after persuading two smaller left-wing parties to back him.

At the time, the alliance was ridiculed as a “geringonca,” or “contraption,” that his opponents said would fall apart in no time–but he has been in power ever since.

The Socialists won an outright majority in snap elections in 2022, giving them enough seats in Parliament to govern without a coalition. The result was seen as a relief for Costa, who had been popular for managing the country’s response to the coronavirus pandemic but also faced questions about his stewardship of the economy, as per The New York Times.