A supervisor saw the chemical being tampered with — as a mouse controlled by the intruder moved across the screen changing settings — and was able to intervene and reverse it

A hacker gained unauthorised entry into a remote access software system shared by workers at a Florida city’s water treatment plant in an unsuccessful attempt to fill the water supply with a potentially harmful chemical, authorities said.

An unknown suspect breached a computer system for the city of Oldsmar's water treatment plant on Friday and briefly increased the amount of sodium hydroxide from 100 parts per million to 11,100 parts per million, Pinellas County Sheriff Bob Gualtieri said during a news conference Monday.

Sodium hydroxide, also called lye, is used to treat water acidity but the compound is also found in cleaning supplies such as soaps and drain cleaners. It can cause irritation, burns and other complications in larger quantities.

A supervisor saw the chemical being tampered with — as a mouse controlled by the intruder moved across the screen changing settings — and was able to intervene and reverse it, Gualtieri said. Oldsmar, a city of 15,000 residents, is about 15 miles (24 kilometers) northwest of Tampa.

“At no time was there a significant adverse effect on the water being treated,” Gualtieri said. “Importantly, the public was never in danger.”

Oldsmar officials have since disabled the remote-access system, and say there were other safeguards to prevent the increased chemical from getting into the water. Officials told other city leaders in the region about the incident and suggested they check their systems.

Experts say municipal water and other systems have the potential to be easy targets for hackers because local governments’ computer infrastructure tends to be underfunded.

Robert M. Lee, CEO of Dragos Security, and a specialist in industrial control system vulnerabilities, said remote access to industrial control systems such as those running water treatment plants has become increasingly common.

“As industries become more digitally connected we will continue to see more states and criminals target these sites for the impact they have on society,” Lee said.

The leading cybersecurity firm FireEye attributed an uptick in hacking attempts it has seen in the last year mostly to novices seeking to learn about remotely accessible industrial systems. Many victims appear to have been selected arbitrarily and damage was not caused in any of the cases, it said in a statement.

Tarah Wheeler, a Harvard Cybersecurity Fellow, said communities should take every precaution possible when using remote access technology on something as critical as a water supply.

“The systems administrators in charge of major civilian infrastructure like a water treatment facility should be securing that plant like they’re securing the water in their own kitchens,” Wheeler told the Associated Press via email. “Sometimes when people set up local networks, they don’t understand the danger of an improperly configured and secured series of internet-connected devices."

A plant worker first noticed the unusual activity at around 8 am Friday when someone briefly accessed the system but thought little of it because co-workers regularly accessed the system remotely, Gualtieri told reporters. But at about 1:30 pm, someone accessed it again, took control of the mouse, directed it to the software that controls water treatment and increased the amount of sodium hydroxide.

The sheriff said the intruder was active for three to five minutes. When they exited, the plant operator immediately restored the proper chemical mix, he said.

Other safeguards in place — including manual monitoring — likely would have caught the change before it reached the water supply, the sheriff said.

Investigators said it wasn’t immediately clear where the attack came from. The FBI, along with the Secret Service and the Pinellas County Sheriff’s Office are investigating the case.

Russian state-backed hackers have in recent years penetrated some US industrial control systems, including the power grid and manufacturing plants while Iranian hackers were caught seizing control of a suburban New York dam in 2013. In no case was damage inflicted but officials say they believe the foreign adversaries have planted software boobytraps that could be activated in an armed conflict.