At least person has been reportedly charged by a Washington grand jury over the U.S. probe into Russia's attempts to tilt the 2016 presidential elections in Donald Trump's favor.

There was no indication, in reporting by CNN that other media later confirmed, of who might be charged or what crimes might be alleged in the ongoing inquiry led by former FBI chief Robert Mueller.

But Trump, in a rapid burst of tweets Sunday, again denounced the investigation as a "witch hunt" and repeated his denials of any collusion with Russia.

Mueller's team has remained mum about reports that a first arrest could be made as early as Monday. He is empowered to pursue not only Russian interference but any other crimes his large team of prosecutors should uncover.

Typically, such an inquiry would first target lower-level people while building a case against those higher up.

Representative Adam Schiff of California, the ranking Democrat on the House intelligence committee, demurred Sunday when asked whether Trump was under investigation. "I can't answer that one way or the other," he told ABC.

But he mentioned two possible targets on whom much speculation has focused: former national security adviser Michael Flynn and former Trump campaign director Paul Manafort, both of them once involved in undeclared lobbying for foreign interests.

Flynn served as security adviser just 24 days after being forced to resign in April over lying to Vice President Mike Pence about his communication with Russian ambassador to the U.S., Sergey Kislyak.

Manafort has been under investigation by the FBI for communication with Russia and conducting financial transactions since 2014. In July, FBI seized documents from Manafort's home reportedly related to Russian meddling in the 2016 presidential election.