A treatment plant in Toledo, Ohio was on alert after a toxin was discovered, leaving 500,000 people without clean tap water.

The high level of toxins was discovered Saturday. The toxins were suspected of coming from a harmful algae that blooms in Lake Erie, reported USA Today.

Mayor Michael Collins warned residents that the drinking water is not yet cleared of the toxins, even though the level of toxins decreased.

“First and foremost, residents must remain calm,” Collins said at a morning press conference.

It was difficult to tell the 400,000 people in the region who began to panic as news of the toxins began to spread. Residents were also told to prevent children from bathing or swimming in the water, for risk of accidentally swallowing.

The water plants along Lake Erie supplies drinking water for 11 million people. In the last few years, there were concerns about the toxins in the water supply. The first reported case of toxins contaminating the water was last year when a township east of Toledo with about 2,000 citizens, was told to not use or bathe in their tap water.

People became frustrated with the water for clean water. Store shelves were left empty and citizens were forced to wait for hours as water was delivered from across the state to Toledo. It was also advised that residents should not boil the water, as it would increase the toxin’s concentration, reported the Washington Post.

“We know it’s difficult. We know it’s frustrating,” said Gov. John Kasich, who declared a state of emergency.

Residents were not told how long the water advisory would last. The reason for the toxins to spike in the drinking water was also unknown.