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Paul Manafort, one of President Trump's campaign managers, and Michael Flynn, Trump's former national security adviser, have become primary figures in the FBI's probe into Russian interference in the 2016 presidential election, several law enforcement officials told NBC News Wednesday.

Over the last six months, as part of the investigation into whether Trump associates colluded with Russians during the campaign, multiple grand jury subpoenas and records requests have been made in connection with Manafort and Flynn, the officials said. Investigators from the FBI, assisted by the CIA, Treasury Department, and other agencies, are specifically looking at evidence of potential business relationships, money transfers, and communication between the two camps. The documents that have been requested include records related to an unusual $3.5 million mortgage Manafort took out on one of his homes in New York directly after he left the Trump campaign.

Both Manafort and Flynn, whose attorneys have said they've done nothing wrong, are considered "subjects" of a criminal investigation, the officials said. On Wednesday, the Justice Department announced that former FBI Director Robert Mueller has been appointed as special counsel and will now oversee the investigation.