Police in Venice are investigating the source of a phosphorescent green liquid patch that appeared Sunday in the city’s famed Grand Canal.

The governor of the Veneto region, Luca Zaia, posted a photo of the green liquid that spread through the water near the arched Rialto Bridge.

The patch was reported by residents.

Images on social media show a bright patch of green in the canal along an embankment lined with restaurants.

Zaia said that officials had requested that the police investigate to determine who was responsible.

Environmental authorities were also testing the water.