NBC News reports two close aides to Donald Trump have emerged as “key figures” in the investigation into Russian collusion with the Trump campaign.

Four law enforcement officials told NBC News that “multiple grand jury subpoenas” have been issued in connection with former Campaign Manager Paul Manafort and former National Security Advisor Michael Flynn.

Deputy Attorney General Rod Rosenstein announced Wednesday that former FBI Director Robert Mueller will head the Justice Department’s investigation into Trump campaign collusion with Russia and Russia interference into the 2016 election campaign.

NBC News also reports that both Manafort and Flynn have been formally categorized as “subjects” of a criminal investigation.

“I hope you can let this go,” James Comey is alleging Trump told him, in reference to the investigation into Michael Flynn.

The investigation is unique, being a hybrid of both a criminal investigation and a counter intelligence probe.

NBC said investigators are examining evidence including contacts, money transfers, business relationships and real estate transactions.