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Portuguese Prime Minister Antonio Costa resigned after his chief of staff was detained as part of an investigation into possible crimes of corruption involving lithium and hydrogen projects.

Costa announced his decision in a speech in Lisbon on Tuesday. His resignation will likely lead to an early election.

The Portuguese Prosecutor-General’s Office said in an emailed statement on Tuesday that Vitor Escaria, the premier’s chief of staff, was among five people detained as part of an ongoing probe related to lithium exploration concessions and a hydrogen production project.

Costa, who’s been prime minister since 2015, currently led a Socialist government backed by an absolute majority in parliament. While he had all that support from lawmakers, over the past year he’s been rocked by other challenges including surging living costs, teachers’ protests and controversies related to state-owned airline TAP SA, which the government plans to privatize.

The prime minister met President Marcelo Rebelo de Sousa in Lisbon earlier. In Portugal, the prime minister and his government set policy. The president is mainly a figurehead, though he has the authority to appoint the premier, dissolve parliament and call elections.

A survey published by Diario de Noticias on October 29 indicated 28.6% support for the Socialists, 3.7 percentage points ahead of PSD, the center-right party that’s the biggest opposition group in parliament. Portugal last held an early election in January 2022.

Searches were carried out on Tuesday in locations including offices used by the premier’s chief of staff as well as the environment ministry and the infrastructure ministry as part of the probe. The prosecutors have also named Infrastructure Minister Joao Galamba as an “arguido,” a status that’s similar to person of interest.

References made by suspects about Prime Minister Costa’s intervention to “unblock” certain procedures will be separately analyzed in an inquiry at the Supreme Court of Justice, the prosecutor said.