About 30 computers containing confidential information on missing Malaysia Airlines Flight MH370 were hacked into, a Malaysian cyber security agency revealed Wednesday.

The hack occurred on March 9, the day after Flight 370 went missing. It is unclear when officials discovered the breach.

Flight 370 vanished while flying between Kuala Lumpur, Malaysia, and Beijing, China. The following day, an email was sent to high-ranking officials investigating the plane's disappearance, claiming that the plane had been found.

Attached to the hacker's email was malware disguised as a PDF file. When the recipients opened the attachment, the cyber thieves were able to access emails, documents, and other information from the targets' computers. A Ministry of Science, Technology and Innovation agency, CyberSecurity Malaysia, confirmed that classified information from the Department of Civil Aviation, the National Security Council, Malaysia Airlines, and other agencies was stolen.

"We received reports from the administrators of the agencies telling us that their network was congested with e-mail going out of their servers," CyberSecurity Malaysia Chief Executive Dr Amirudin Abdul Wahab told The Star.

"Those e-mail contained confidential data from the officials' computers, including the minutes of meetings and classified documents. Some of these were related to the MH370 investigation."

The data was being sent to an IP address in China.

"This was well-crafted malware that antivirus programs couldn't detect," Amirudin added. "It was a very sophisticated attack."

The CyberSecurity officials asked the internet service provider to block the IP address, and the agency, Malaysian police, and Interpol are investigating the incident.

Dr Amirudin said the motivation for the cyber attack may have been the perception that officials were withholding information about the missing plane.

"At that time, there were some people accusing the Government of not releasing crucial information," he said. "But everything on the investigation had been disclosed."

No evidence of debris from Flight 370 or a crash site have been discovered. There were 239 people aboard the plane.