Robert Mueller have recently been asking witnesses about Donald Trump's business activities in Russia prior to the 2016 presidential campaign as he considered a run for president, according to three people familiar with the matter.

Questions to some witnesses during wide-ranging interviews included the timing of Trump's decision to seek the presidency, potentially compromising information the Russians may have had about him, and why efforts to brand a Trump Tower in Moscow fell through, two sources said.

The lines of inquiry indicate Mueller's team is reaching beyond the campaign to explore how the Russians might have sought to influence Trump at a time when he was discussing deals in Moscow and contemplating a presidential run.

Mueller was appointed to investigate Russian meddling in the 2016 campaign. The President claims that any investigation of his family's finances would be a breach of the special counsel's mandate.

Two of the sources said they do not know from the questions asked whether Mueller has concrete evidence to indicate wrongdoing.

"You ask everything even if you don't think it's credible," one of the sources said, adding, "the allegations are out there, and it was checking the box."