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The Pentagon has banned, at least temporarily, the use of external computer flash drives because of a virus threat officials detected on Defense Department networks.

While defense officials would not publicly confirm the ban, messages were sent to department employees informing them of the new restrictions. As part of the ban, the Pentagon was collecting any of the small flash drives that were purchased or provided by the department to workers, according to one message distributed to employees.

Workers are being told there is no guarantee they will ever get the devices back and it is not clear how long the ban will last.

Pentagon spokesman Bryan Whitman would provide no details on the virus Friday, but he described it as a "global virus" that has been the subject of public alerts.

"This is not solely a department problem, this is not solely a government problem," Whitman said.

The Pentagon has acknowledged that its vast computer network is scanned or probed by outsiders millions of times each day. Last year a cyber attack forced the Defense Department to take up to 1,500 computers off line.

Officials said then that a penetration of the system was detected, but the attack had no adverse impact on department operations.

However, military leaders have consistently warned of potential threats from a variety of sources including other countries -- such as China -- along with other self-styled cyber-vigilantes and terrorists.

The issue has also been of concern at the Department of Homeland Security. A September audit by the DHS Inspector General recommended that the agency implement greater procedures to ensure that only authorized computer flash drives or other storage devices can be connected to the network there and that an inventory of those devices be set up.

DHS agreed with the recommendations and said some of that is already being done. DHS also said more software enhancements are in the works that will provide more protection.

WASHINGTON (CNN) -- Computers at the headquarters of the Barack Obama and John McCain campaigns were hacked during the campaign by a foreign entity looking for future policy information, a source with knowledge of the incidents confirms to CNN.

Workers at Barack Obama's headquarters first thought there was a computer virus.

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The source said the computers were hacked mid-summer by either a foreign government or organization.

Another source, a law enforcement official familiar with the investigation, says federal investigators approached both campaigns with information the U.S. government had about the hacking, and the campaigns then hired private companies to mitigate the problem.

U.S. authorities, according to one of the sources, believe they know who the foreign entity responsible for the hacking is, but refused to identify it in any way, including what country.

The source, confirming the attacks that were first reported by Newsweek, said the sophisticated intrusions appeared aimed at gaining information about the evolution of policy positions in order to gain leverage in future dealings with whomever was elected.

The FBI is investigating, one of the sources confirmed to CNN. The FBI and Secret Service refused comment on the incidents. Watch Brian Todd's report on the investigation. »

The sources refused to speak on the record due to the ongoing investigation and also because it is a sensitive matter involving presidential politics.

As described by a Newsweek reporter with special access while working on a post-campaign special, workers in Obama's headquarters first detected what they thought was a computer virus that was trying to obtain users' personal information.

The next day, agents from the FBI and Secret Service came to the office and said, "You have a problem way bigger than what you understand ... you have been compromised, and a serious amount of files have been loaded off your system."

One of the sources told CNN the hacking into the McCain campaign computers occurred around the same time as the breach into those of Obama's campaign.

Representatives of the campaigns could not be reached for comment on the matter.