By Sam Frizell, TIME

Sony Pictures Entertainment could not have been “fully prepared” for a massive hack that hit the company last month, the head of a cybersecurity firm hired by Sony said in an internal memo published Monday.

In an internal email obtained byRe/code, Sony CEO Michael Lynton shared a note with employees from Kevin Mandia, head of security firm Mandiant, that called the Sony (SNE) hack an “unparalleled crime” carried out by “an organized group.” Sony has contracted with Mandiant to investigate and clean up the breach.

“The malware was undetectable by industry standard antivirus software and was damaging and unique enough to cause the FBI to release a flash alert to warn other organizations of this critical threat,” said Mandia in the note to Lynton. It went on to say that “neither SPE nor other companies could have been fully prepared” for the attack, which leaked employees’ salaries, social security numbers and other data, as well as unreleased films.

The Mandiant letter may aim to vindicate Sony from responsibility for the hack after the company was accused of showing a cavalier attitude toward cybersecurity. Only 11 people were assigned to Sony’s security team, Fusion reported last week, while Sony Pictures’ executive director of information security Jason Spaltro told CIO Magazine in 2007 that it may be “a valid business decision to accept the risk” of a security breach.

Here’s the full note from Re/code:

Over the last week, some of you have asked about the strength of our information security systems and how this attack could have happened. There is much we cannot say about our security protocols for obvious reasons, but we wanted to share with you a note we received today from Kevin Mandia, the founder of the expert cybersecurity firm that is investigating the cyber-attack on us. The investigation is ongoing, but Mr. Mandia’s note is helpful in understanding the nature of what we are dealing with. Full text below.

We also want to thank you once again for your resilience and resourcefulness in carrying out our critical day-to-day activities under incredibly stressful circumstances. As a result of your efforts, we have made great progress moving our business forward, and we will continue to do so.

— — —
Dear Michael,

As our team continues to aid Sony Pictures’ response to the recent cyber-attack against your employees and operations, I wanted to take a moment to provide you with some initial thoughts on the situation.

This attack is unprecedented in nature. The malware was undetectable by industry standard antivirus software and was damaging and unique enough to cause the FBI to release a flash alert to warn other organizations of this critical threat.

In fact, the scope of this attack differs from any we have responded to in the past, as its purpose was to both destroy property and release confidential information to the public. The bottom line is that this was an unparalleled and well planned crime, carried out by an organized group, for which neither SPE nor other companies could have been fully prepared.

We are aggressively responding to this incident and we will continue to coordinate closely with your staff as new facts emerge from our investigation.

Sincerely,
Kevin Mandia