New Orleans has declared a state of emergency after falling victim to a cyber attack which forced the shutdown of all the city government’s computers.
The attack, which was first detected at 5 am on Friday, intensified as staff logged on for work about three hours later.
Officials detected ransomware and phishing attempts, forcing the city to take all its computers offline at 11 am on Friday.
Despite a flood of suspicious emails, there was no indication that passwords were compromised or data stolen said the city’s chief information officer, Kim LaGrue.
It was unclear whether any of the ransomware had succeeded in blocking staff access to their computers.
"Out of an abundance of caution, all employees were immediately alerted to power down computers, unplug devices and disconnect from WiFi," the city said on its Facebook page.
Staff reverted to using pen and paper as they sought to keep services running in the city which has a population of just under 400,000.
The New Orleans city website remained shut for maintenance on Sunday.
Despite the attack, New Orleans, building on its experience of coping with disasters like Hurricane Katrina, was able to function without the internet.
Police and emergency services relied on the radio network to keep operating.
Only last month the state of Louisiana was targeted by cyber criminals who succeeded in shutting down the state’s office of motor vehicles, which is responsible for issuing driving licences, for several weeks.
Ransomware attacks, in which criminals try to extort money by blocking access to computers, have also been reported by authorities in Texas, North Carolina, California, Maine and Florida.
In April the main airport in Cleveland Ohio was hit by malware which put flight and luggage information boards out of action for five days.
Repairing networks is costly. Atlanta, which was crippled by SamSam ransomware in March last year, reportedly spent $2.6 million fixing the damage to its systems.
According to the FBI, the US is hit by 4,000 ransomware attacks a day with criminals using malware capable of defeating firewalls and traditional anti-virus software.