A mysterious patch of fluorescent green water that appeared in Venice's famed Grand Canal Sunday was caused by a chemical commonly used in underwater construction to help identify leaks, environmental authorities say.

The chemical -- fluorescein -- is non-toxic. It remains unclear how the substance ended up in the canal, but the Regional Agency for the Environment in Venice (ARPAV) said given the volume released it was unlikely to be an accident.

The verdant blob was first noticed by residents near the Rialto Bridge on Sunday morning local time and grew slowly through the day.

Images showed gondolas, water taxis and water bus boats skimming through the emerald substance.

No group has claimed responsibility for the act and local police are investigating a number of leads, including environmental activism, a spokesperson for the Venice Police told CNN.

Further test results are expected later this week, which could help identify the exact quantity of the substance in the water.

Luca Zaia, the president of the region of Venice, warned that environmental activists may carry out copycat acts.

Further test results are expected later this week, which could help identify the exact quantity of the substance in the water.

The curious coloring came as the city celebrated the Vogalonga boat event, created to combat wave motion and to restore Venetian traditions and help spread attention for the environment and nature as well as the architecture Biennale, which opened last weekend.

This is not the first time Venice's Grand Canal has changed color.

In 1968 Argentine artist Nicolás García Uriburu dyed the waters of the canal green with a fluorescent dye called Fluorescein, during the annual Venice Biennale. The move was designed to bring attention to ecological issues and the relationship between nature and civilization.