A group of black Oakland Public Works employees noticed a noose was placed on one of their trucks Tuesday–in a department in Northern California where many say racial tensions have been brewing. 

According to San Francisco’s local CBS News, KPIX, the workers’ union says the hangman’s noose is representative of racism in the department. Mayor Jean Quan reluctantly told KPIX that she thought there weren’t racial tensions there, only concerns over promotions.

The case is being investigated by Oakland Police Department.