MEXICO CITY -- State police have detained and disarmed the entire police force of a town in western Mexico where a mayoral candidate was killed last week. Video of the detention aired by local media showed uniformed officers hitting each other as gunshots go off in the background.

The Michoacan state police force said the "all the officers of the Ocampo municipal police force were detained for internal affairs investigation."

In statements on its Twitter account Sunday, the state police department did not directly tie the detentions to the Thursday killing of Fernando Angeles Juarez, the mayoral candidate for the leftist Democratic Revolution Party.

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In 2010, the mayor of the northern Mexico town of Santiago, Nuevo Leon was killed by his own police in retaliation for his attempts to oust corrupt officers.

Ocampo is a rural township about 95 miles west of Mexico City best known for the Monarch butterfly wintering grounds that occupy part of the mountainous municipality. It also been plagued by illegal logging and gangs.

Almost all of the 18 candidates killed across the country so far have been running for local posts in the July 1 elections, which will also decide the presidency, governorships and Congress. Other politicians who were considering a run have been killed before they could even register as candidates. The killings have particularly hit states like Michoacan, Guerrero and Oaxaca.

Politicians are being targeted in Mexico ahead of the country's July elections, and one murder earlier this month was caught on surveillance video, CBS News correspondent Manuel Bojorquez reported. The footage shows Mexican congressional candidate Fernando Puron posing for a selfie with a supporter after a campaign debate last Friday, when a man walks up from behind, raises a gun and fires at the back of his head.