MEXICO CITY (Reuters) - Twenty-eight police officers in the western Mexican state of Michoacan have been arrested on suspicion of involvement in the murder of a candidate for mayor in the municipality of Ocampo, local media reported on Sunday.

State authorities issued a statement on Sunday saying all members of Ocampo’s municipal police department had been taken in for questioning by the police’s internal affairs department.

The probe focused on potential violations of the police code of conduct, the statement said, without giving more details.

Local and national media said the officers were held on suspicion of complicity in the killing of Fernando Angeles Juarez, who was running for mayor of Ocampo, which lies on the eastern fringe of Michoacan and is home to some 24,000 people.

Juarez was shot dead on Thursday, and was competing for the center-left Party of the Democratic Revolution (PRD), which governs the long-troubled state.

A source in the state familiar with the investigation said that the media reports about the arrests were correct. Twenty-eight police were detained in the operation, the source added, speaking on condition of anonymity.

Mexico will vote for a new president on July 1 alongside hundreds of other federal, state and municipal posts.

The 2018 election campaign has been the bloodiest in Mexico’s modern history with dozens of politicians, candidates and activists murdered by criminal groups and gangsters seeking to influence the shape of the post-electoral map.