Riley June Williams was taken into custody in the Middle District of Pennsylvania on Monday.
The FBI said in an arrest warrant Sunday, however, that Williams hasn't been charged with theft but only with illegally entering the Capitol and with disorderly conduct.
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FBI officials said a caller claiming to be an ex of Williams said friends of hers showed him a video of her taking a laptop computer or hard drive from Pelosi's office. The caller alleged that she intended to send the device to a friend in Russia who planned to sell it to that country's foreign intelligence service, but that plan fell through and she either has the device or destroyed it. The FBI says the matter remains under investigation.
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Pelosi's deputy chief of staff, Drew Hammill, confirmed Jan. 8 that a laptop was taken from a conference room but said "it was a laptop that was only used for presentations."
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Williams' mother, who lives with her in Harrisburg, told ITV reporters that her daughter had taken a sudden interest in President Donald Trump's politics and "far-right message boards." Her father, who lives in Camp Hill, told local law enforcement that he and his daughter went to Washington on the day of the protest but didn't stay together, meeting up later to return to Harrisburg, the FBI said.
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FBI officials said they believe Williams "fled" before she was arrested. Her mother told local law enforcement that she packed a bag and left, saying she would be gone for a couple of weeks. She also changed her phone number and deleted a number of social media accounts, the FBI said. Court documents don't list an attorney for her.
-- The Associated Press contributed to this report.