A shocking case of computer hacking has been uncovered in Pinellas County, Florida. Federal investigators are trying to hunt down the person who tried to poison a public water supply — remotely.

Investigators say a plant operator monitoring the water plant in the Tampa Bay city of Oldsmar noticed breaches starting Friday morning.

The hacker was controlling the computer system’s mouse — opening various functions on the screen and changing the sodium hydroxide in the water supply from about 100 parts per million to more than 11,100 parts per million.

“This is obviously a significant and potentially dangerous increase,” Pinellas County Sheriff Bob Gualtieri said. “Sodium hydroxide, also known as lye, is the main ingredient in liquid drain cleaners.”

If ingested in large amounts, sodium hydroxide can cause vomiting, chest and abdominal pain, according to the Centers for Disease Control.

Fortunately, inside the water treatment plant the plant operator immediately reduced the levels back to what was safe.

Now Secret Service and FBI cyber units are trying to determine who is behind the hack and whether it was someone in the U.S. or overseas. It occurred just two days before the Super Bowl in a city about 30 miles away from Raymond James Stadium.

“But the important thing is to put everybody on notice … This is kind of bad. Actors are out there, it’s happening,” Oldsmar Mayor Eric Seidel said.

Cyber experts warn that hackers have gotten into infrastructure before — but doing something dangerous — changes the calculus.