Police in Ukraine have seized the servers of an accountancy software firm, which is believed to have unwittingly helped spread malware that attacked many global firms last month.
Intellect Service has denied that its software helped spread the malware.
But security experts have said that some of the initial infections were indeed spread via a malicious update to MeDoc.
It is Ukraine’s most popular accounting software.
The cyber-attack – a variant of an earlier virus called Petya – hit businesses around the world including the shipping firm Maersk and the marketing giant WPP.
It was initially thought to be a ransomware attack designed to make money for the hackers behind it.
But some security firms now think that it was deliberately designed to destroy data and targeted Ukraine.
The head of the country’s national Cyberpolice unit had previously alleged that Intellect Service had ignored repeated warnings that it needed to improve its security in advance of the attack.
“They were told many times by various anti-virus firms,” Col Serhiy Demydiuk told the Associated Press news agency.
“For this neglect, the people in this case will face criminal responsibility.”
In a separate interview with Reuters, the father and daughter team who run Intellect Service said they were not responsible.
“What has been established in these days, when no one slept and only worked? We studied and analysed our product for signs of hacking – it is not infected with a virus and everything is fine, it is safe,” said Olesya Linnik, managing partner at Intellect Service.
“The update package, which was sent out long before the virus was spread, we checked it 100 times and everything is fine.”
Security experts, including Microsoft, Cisco and Symantec, said that they all have evidence that the malware was spread via an update to the tax software program MeDoc.
It is believed that around 80% of companies in Ukraine use the software, which allows clients to send and discuss financial documents internally as well as file them with the government’s tax department.
Ukraine police said that the family could face criminal charges if it is confirmed that they knew about the infection but took no action.