Ukrainian police on Tuesday seized the servers of an accounting software firm suspected of spreading a malware virus that crippled computer systems at major companies around the world last week, a senior police official said.

The head of Ukraine's Cyber Police, Serhiy Demedyuk, told Reuters the servers of M.E.Doc — Ukraine's most popular accounting software — had been seized as part of an investigation into the attack.

Though they are still trying to establish who was behind last week's attack, Ukrainian intelligence officials and security firms have said some of the initial infections were spread via a malicious update issued by M.E.Doc, charges the company's owners deny.