Hill has been subpoenaed and is working with her lawyer on logistics.
She officially departed her role in August, though she had handed over most of her responsibilities in July, but was involved as Rudy Giuliani was making public pronouncements about Ukraine.
Hill has kept a low profile in recent weeks in England with her mother.
CNN has reached out to her attorney.
There is no evidence of wrongdoing by either Joe or Hunter Biden.
Hill, a former Brookings scholar and skeptic of Russian President Vladimir Putin, oversaw rocky Washington-Moscow ties while she held her role.
Hill’s decision to serve on Trump’s NSC was viewed as an intriguing move at the start of the administration.
A critical biographer of Putin, Hill’s views sometimes seemed at odds with Trump’s own desire to improve relations with the strongman leader who Trump, as a candidate, often spoke of admiringly.
This story has been updated.