MILAN — Police in Venice are investigating the source of a slick of phosphorescent green liquid that appeared on the city’s famous Grand Canal on Sunday.

The governor of the Veneto region, Luca Zaia, posted a photo of the green liquid that spilled into the water near the arched Rialto bridge.

The patch was initially reported by residents. Images on social media show a bright green patch in the canal along an embankment lined with restaurants.

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

Environmental authorities were also testing the water.