Two heads and other human remains were left at polling stations in the Mexican border city of Tijuana during elections on Sunday, June 6, authorities reported.

An hour after elections got underway, a man hurled a head at one polling station, interrupting the vote as the police were called, prosecutors in the state of Baja California said.

A couple of hours later, at another polling station in the same area, a man left another head and dismembered human remains inside a wooden box placed next to the ballot boxes.

More human remains were discovered in bags near a third polling station, according to the Baja California Prosecutor’s Office.

Since the elections began in September, there has been a surge of political violence that has resulted in the deaths of over 90 lawmakers.

It is not the first time that Tijuana has witnessed gruesome discoveries of human remains, which have previously included mutilated bodies hanging from bridges.

Across Mexico, more than 300,000 people have been killed in a wave of bloodshed since the government deployed the army to fight drug cartels in 2006.