(CNN) The entire police force of a Mexican town in the state of Michoacán is being questioned in the assassination of a mayoral candidate, the office of the state's attorney general told CNN on Monday.

Fernando Angeles Juarez of the Party of the Democratic Revolution was killed Thursday morning by unknown gunmen.

On Saturday, Michoacán State Police went to Ocampo to question 28 municipal officers in a routine check unrelated to Juárez's death, the law enforcement agency said on Sunday. When they arrived, some of those officers fired shots in the air and resisted their authority, Michoacán State Police said. The agency said all 28 officers were detained on Sunday.

On Monday, the Michoacán State Attorney General's Office confirmed that the officers are now under investigation in Juárez's death.

Mexico is due to go to the polls on July 1 to pick its next president as well as hundreds of federal, state and municipal positions.

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