Story highlights The FBI is investigating a possible hack of Democratic Party staffer cell phones

Johnson told Congress that 18 states have asked for help defending against cyberattacks

Washington (CNN) The FBI has asked to examine the cell phones of a small number Democratic Party staffers as it investigates a possible hack, law enforcement and Democratic sources told CNN Tuesday.

The development comes on the same day Homeland Security Secretary Jeh Johnson told lawmakers that 18 states have asked for help in warding off cyberattacks on their electronic voting systems.

Law enforcement officials have reached out to the staffers individually about "imaging" their phones to search for evidence of hacking, such as malware. Investigators are still probing whether this attempted hack is part of the original breach of Democratic National Committee emails -- which is widely thought to be the work of the Russian government -- or a new hacking attempt.

"Our struggle with the Russian hackers that we announced in June is ongoing -- as we knew it would be -- and we are choosing not to provide general updates unless personal data or other sensitive information has been accessed or stolen," interim DNC Chairwoman Donna Brazile told CNN.

Meanwhile, Johnson outlined to the Senate Homeland Security Committee the concerns states had about defending their voting systems from hackers.

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