Robert Mueller. Thomson Reuters

The special counsel Robert Mueller enlisted the IRS' criminal investigations unit earlier this month as part of his inquiry into possible collusion between the Trump campaign and Russia — but the unit has only just begun to share information about key players like Paul Manafort and Michael Flynn, according to CNN.

The FBI teamed up with the IRS before the election to investigate Manafort, President Donald Trump's former campaign chairman, on suspicion of money laundering and tax fraud, according to CNN. But the IRS was apparently irked when Mueller's team obtained a warrant to raid Manafort's Virginia home in late July in search of bank records and other financial documents.

The IRS reportedly sat out the raid because it feared the raid could hinder its own investigation into Manafort's tax dealings, people briefed on the investigation told CNN.

Title 26 US Code limits what the IRS can share with Mueller, especially as it relates to tax returns. It is still unclear whether Mueller has Trump's tax returns, but legal experts have said the fact Mueller enlisted the criminal-investigations unit, known as IRS-CI, in the first place indicates he spotted something of interest in someone's tax returns — which federal investigators typically obtain early — that he believes is worth pursuing.

"A federal prosecutor can bring in IRS-CI whenever he or she is investigating tax charges," said Renato Mariotti, a former federal prosecutor in the Securities and Commodities Fraud Section of the US attorney's office.

Mariotti said tax returns tell federal prosecutors who a person of interest owes money to or is owed money by, making the returns invaluable not only in identifying potential conflicts of interest but also in revealing potential tax crimes. That Mueller has enlisted the IRS-CI, Mariotti wrote earlier this month, "very strongly indicates" that he has uncovered evidence of a tax crime and is investigating it.

"If a federal prosecutor wants to investigate a tax crime, he or she must involve the IRS," Mariotti said. "You can't charge tax cases without them."

The IRS has sole jurisdiction over pure tax crimes. But when an investigation moves past that, there are crimes, like money laundering, that the IRS works with the FBI on.