The spectacular transformation of a stretch of Venice's Grand Canal to fluorescent green was due to fluorescein, a non-toxic substance used for testing wastewater networks, local authorities said on Monday.
Residents noticed a stretch of Venice's Grand Canal turned bright green Sunday, prompting police to investigate amid speculation it could be a stunt by environmentalists.
But analysis showed "the presence of fluorescein in samples taken", said the the Regional Agency for Environmental Prevention and Protection of Veneto (Arpav).
The results "have not shown the presence of toxic elements in the samples analysed", the statement said, without specifying the origin of the substance.
The change in colour noticed by residents raised eyebrows, with police looking into whether Sunday's development could be a protest by climate change activists, according to local daily La Nuova Venezia.
It is not the first time the Grand Canal has turned green.
In 1968, Argentine artist Nicolas Garcia Uriburu dyed the waters of Venice's Grand Canal green with a fluorescent dye during the 34th Venice Biennale in a stunt to promote ecological awareness.