A recent cyberattack on a Florida city’s water treatment plant attempted to pump up the amount of lye, which is poisonous in large doses, in the city’s water supply, officials said.

The cyberattack on the city of Oldsmar’s water treatment plant on Feb. 5 momentarily increased the amount of sodium hydroxide, or lye, added to the city’s water supply from 100 parts per million to 11,100 parts per million. An alert plant worker saw the hacker’s efforts happening live on a computer’s desktop and immediately changed the lye levels back to safe levels, Pinellas County Sheriff Bob Gualtieri said in a press conference on Feb. 8.