The first damage assessment of a sprawling cyber attack believed to be linked to Russia has been chilling enough. 

With intrusions reported across a huge swath of the government – including at the Department of Energy's National Nuclear Security Administration – federal officials already are signaling that the worst may be yet to come.

The Department of Homeland Security's cyber security unit has acknowledged that the full scope of the attack is not yet known, with an untold number of local government and private sector systems at "grave risk."

Although federal authorities have so far traced the attack's launch back to March, it remains unclear just how long operatives have been lurking in some of the government's most critical agencies – including the departments of State, Homeland Security, Treasury and Commerce –and what may have been lost or compromised.