The State Department has shut down its unclassified e-mail system after it became the latest victim of hackers targeting the U.S. government.
The FBI is now investigating at least four cyber attacks that have infiltrated key government systems over the past several weeks. Sources say Chinese hackers targeted National Weather Service computers and the personal data of 800,000 U.S. Postal workers.
Around the same time, cyber spies thought to be from Russia hit unclassified email networks at the White House and the U.S. State Department.
Officials say there is no evidence any classified information was compromised. Shawn Henry, who led the FBI's cyber division, and who now runs the security firm Crowdstrike, says the break-ins appear to be spy operations.
"This is most likely an intelligence collection operation," said Henry. "They are looking to gather intelligence about who the players are within the government, who they are communicating with, etc., and the new initiatives they are developing.
Henry said the spate of breaches might indicate that U.S. businesses and agencies are getting better at detecting intrusions. But preventing them is a bigger challenge.
"I think our best way to get through this is to manage this, to be able to detect it quickly and then to mitigate it not unlike how we treat major diseases. We are not going to eradicate them. We have to detect them and we have to respond and mitigate the consequences," he said.
But can the break-ins be stopped?
"I don't think that we can, absolutely not," said Henry.
Billions have been spent to better protect U.S. computer systems. But analysts warn there is no fail-safe network that's beyond the reach of highly motivated cyber thieves.