An anonymous Defense Department official claims hackers infiltrated an e-mail system at the Pentagon late last month. The system, used by roughly 4,000 personnel that work for the Joint Chiefs of Staff, is of unclassified nature according to sources as reported by NBC News.

The attack appears to have used an automated system that gathered a heap of data and rapidly distributed it to thousands of accounts across the Internet. It is believed that the attackers coordinated with each other using encrypted social media accounts.

The e-mail system was taken offline as experts continue to analyze the attack to determine the scope and exactly who might have been behind it. Early indications point to the Russian government although it’s unclear if it was sanctioned directly by the government or carried out by rogue individuals.

Cyber attacks emanating from Russia are a common occurrence and have been for years. In late 2013, a well-known Russian hacker infiltrated a BBC server and tried to sell access to other hackers in an underground forum.

A year ago, it was revealed that a small Russian hacking collective had built up a massive database of 1.2 billion usernames and passwords. The bad actors used SQL injection attacks to gather credentials from around 420,000 sites.

The Pentagon e-mail system is expected to go back online soon.