The water on Venice’s Grand Canal turned phosphorescent green on Sunday near the Rialto Bridge, and authorities are trying to determine the cause, Italian firefighters said, Reuters reported.

The regional environmental protection agency received samples of the discolored water and is working to identify the substance that caused the change, the agency said in a tweet.

The prefect of Venice called an emergency police meeting to understand what happened and to study possible countermeasures, the Ansa news agency reported.

The incident follows a series of recent episodes in Italy where environmental groups painted over monuments, including using charcoal to blacken the waters of Rome’s Trevi Fountain, in protest against fossil fuels.

However, unlike in previous cases, no activist group came forward to claim responsibility for what happened in Venice.