Calls are mounting in Serbia for an independent investigation into reports that security forces used a sonic weapon on crowd at against a huge anti-corruption rally last weekend.

Amidst vehement denials by authorities, calls are rising for an independent investigation into claims that a prohibited sonic weapon was used on protesters at an anti-corruption rally in Serbia. Allegations of the weapon’s use continue to ignite political tension,

The protests were ignited following the deaths of 15 people, who were killed when a concrete canopy collapsed at a train station in Serbia in November, CNN reported. The crowd was having a 15-minute moment of silence when the sonic weapon is alleged to have been used, the Sarajevo Times reported.