More than 500,000 Ohio residents were left without drinking water after a dangerous toxin was discovered at a water-treatment plant in northwestern Ohio, for a second consecutive day, according to reports.

D. Michael Collins, the mayor of Toledo, said that preliminary results of water samples sent for testing showed that toxin levels are falling but added that an official analysis by federal authorities would be required before local officials could declare the water safe to drink. Algae found in Lake Erie, which is the main source of water to the city, is the culprit for the toxins in the water, Collins said.

"This is not over yet," Collins said, according to a report Sunday.

After news of the contamination was made public on Saturday, residents of Toledo along with those living in southeastern Michigan began to line up for water. The National Guard reportedly supplied 33,000 gallons of drinking water to residents of affected areas on Sunday with an additional 15,000 gallons delivered in collapsible containers.

Ohio Governor John Kasich declared a state of emergency for Lucas, Fulton and Wood counties, and officials advised people not to drink boiled water as the boiling process would only concentrate the toxins present in the water, making it even more dangerous for consumption.

Preventing another algae attack will be the next priority for officials, once the water is declared safe to drink.

Stores as far as 50 miles away from Toledo began reporting a shortage of bottled water once the news of the toxic water broke. Residents of affected areas had the option of getting potable water from half a dozen water-filling stations, while vendors reportedly pitched in to give away drinking water.

The toxins were detected when chemists working at Toledo's Collins Park Water Treatment Plant tested water samples and found microcystin -- a toxin produced by algae blooms -- in two readings far above the one-microgram per liter limit.

Immediately after the discovery, authorities warned people of the toxic nature of the water and advised people not to drink it, adding that healthy adults could use the water to bathe but advised against children from doing so, to prevent accidental ingestion.

Algae infestation of Lake Erie reportedly created a similar problem last year with residents of Carrol Township being affected for several days.