US investigators originally thought that the attack on government agencies and private industry targets began in March or April, including breaches of Treasury, State, Commerce and Energy Departments. State-backed Russian hackers were identified as the suspects. Russia has denied involvement.
"The initial burrowing in may have started earlier," Democratic Senator Mark Warner of Virginia, who serves as Vice-Chair of the Senate Intelligence Committee told Reuters in an interview.
Warner said extensive investigations of the hack were active but that so far the US government does not have hard evidence that classified government secrets were compromised by the hackers.