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President Trump took to Twitter on Friday to vent his frustration over the widening investigation into ties between his campaign and Russia — in the process, confirming that he was a subject of the investigation.

Trump also mocked the probe, tweeting that "after 7 months of investigations [and] committee hearings about my 'collusion with the Russians,' nobody has been able to show any proof. Sad!"

White House aides are reportedly concerned that "the greatest threat to Trump and his presidency ... comes from his own conduct and obsessive behavior after he took office," Politico writes.