Special counsel Robert Mueller has enlisted the help of the IRS criminal investigations unit for the FBI's Trump-Russia investigation.

The agents, whom the Daily Beast reports Mueller has great respect for, exclusively focus on financial crimes such as tax evasion and money laundering.

The IRS has access to Trump's tax returns, which he has refused to release to the public.

Mueller's investigation, which began in May, is looking into possible financial crimes committed during the 2016 election. His team has partnered with New York Attorney General Eric Schneiderman as it investigates financial crimes Trump's former campaign chairman Paul Manafort may have committed.

Mueller's Russia probe is looking into a number of former campaign officials as the investigation looks at possible collusion between the Trump campaign and Russia.