Milan regional officials said the cause was certainly sabotage at a former refinery turned oil depot, since the cisterns were opened and the oil allowed to flow unimpeded into the Lambro River near Monza.
The cisterns "were opened by someone who was familiar with the plant and knew how to operate them," said Cinzia Secchi, a spokeswoman for the Milan provincial government.
There were varying accounts of the amount of oil released: Secchi said officials now believed 2.5 million liters (660,000 gallons) had poured out, down from the initial estimates of 10 million liters but significantly more than the 600,000 liters reported by the ANSA news agency and environmental groups.
Environmentalists warned that several water and bird species were at risk from the spill, since the area is rich in bird and other wildlife. But even after the spill is cleaned up the impact will last as the Po river valley is the most important agricultural region in Italy, and the Po is used extensively for irrigation, the World Wildlife Fund for Nature noted.
The spill began Tuesday and spread south down the Lambro to Piacenza and Cremona overnight, despite efforts to contain it. By Wednesday, it had reached the Po, which crosses the country from Piedmont in the west, across Turin and Ferrara before emptying into the Adriatic sea.
The 130 kilometer (80-mile) Lambro — a tributary to the Po that means "clear" in Latin — had been polluted by years of industrial runoff well before the spill.
But Damiano di Simine, regional president of the Legambiente environmental group, said the slick had caused even more damage to a tributary that had just recently shown signs of recovery, with fish returning.
"The scale of this is dramatic," he said in a phone interview, noting that Legambiente — as well as the regional government — had asked that a state of emergency be declared to free up federal funds to help contain it.
"We don't yet know the details, but there is great damage to the ecological system — all the vegetation and fauna," he said.
Several oil-covered ducks have already been plucked from the river and taken for treatment at a regional animal shelter.
The WWF said most at risk were fish, wild ducks and herons, who in these days were beginning to nest along the Po.
The Lombard regional president, Roberto Formigoni, said those responsible would be prosecuted and punished severely for what his office called an "ecological disaster."
"Some criminal decided to intervene in a harmful and cowardly way, putting at risk an asset that belongs to all of us," the Apcom news agency quoted Formigoni as saying. "It's an act of hatred that will be punished by everyone."
While no arrests have been made, Italian news reports have noted that the depot owner, Lombarda Petroli, had laid off several workers in recent months as it downsized. There was no answer at the company Wednesday.
Di Simine charged that the company had in the past year managed to get off a list of at-risk industrial plants, which would have required it to keep up safety standards and an emergency contingency plan to deal with a spill.