Federal prosecutors in Manhattan are considering criminal charges against former White House counsel Greg Craig, who served during the first year of the Obama administration, as part of an investigation into former Trump campaign chairman Paul Manafort, according to a new report.

Sources familiar with the matter told CNN that prosecutors in the U.S. Attorney’s Office for the Southern District of New York are investigating whether Craig failed to register as a foreign agent. They’re also weighing action against law firm Skadden, Arps, Slate, Meagher & Flom LLP, where Craig was employed during the activity under investigation, the report said.

An attorney for Craig told CNN his client “was not required to register under the Foreign Agent Registration Act.”

The charges were referred to the U.S. Attorney’s office by special counsel Robert Mueller’s team.

The investigation relates to whether Craig lobbied on behalf of a group linked to Ukraine without registering with the Justice Department as a foreign agent, which is required under federal law.

Details about the matter were included in superseding criminal charges filed Friday by Mueller’s office against Manafort. As part of a plea deal reached Friday, Manafort agreed to cooperate with the federal government.

The filing alleged the law firm was “solicited” by Manafort on behalf of then-President of Ukraine Viktor Yanukovych and the Ukranian government's Ministry of Justice to write a report on the trial of a political rival of Yanukovych. Craig was the law firm’s partner who worked on the report and was involved in disseminating it to members of Congress and the media.