( CNN) Jared Kushner has turned over documents in recent weeks to special advise Robert Mueller as investigators have begun asking in witness interviews about Kushner's role in the firing of FBI Director James Comey, CNN has learned.

Their questions about Kushner signal that Mueller's researchers are reaching the President's inner circle and have widened beyond the 2016 campaign to actions taken at the White House by high-level officials. It is not clear how Kushner's advice to the President might relate to the overall Russia investigation or potential blockage of justice.

Sources close to the White House say that based on their knowledge, Kushner is not a target of the investigation.