About 200 US businesses have been hit by a "colossal" ransomware attack, according to a cyber-security firm.
Huntress Labs said the hack targeted Florida-based IT company Kaseya before spreading through corporate networks that use its software.
Kaseya said in a statement on its own website that it was investigating a "potential attack".
Huntress Labs said it believed the Russia-linked REvil ransomware gang was responsible.
The US Cybersecurity and Infrastructure Agency, a federal agency, said in a statement that it was taking action to address the attack.
The cyber-breach emerged on Friday afternoon as companies across the US were clocking off for the long Independence Day weekend.
The two big things that are keeping cyber-security professionals up at night lately are ransomware attacks and supply chain attacks. This latest incident combines both nightmares into one big Independence Holiday weekend-ruining event for hundreds of US IT teams.
Ransomware is the scourge of the internet. Multiple organised criminal gangs are constantly attempting to gain access to computer networks to hold them hostage. The rate of attack is relentless but it can take a lot of time and effort on the criminals part to successfully hijack one victim's computer system.