If special counsel Robert Mueller finds any crime outside the scope of the federal investigation into Russian interference in the 2016 election, then he'll have to seek permission, Deputy Attorney General Rod Rosenstein said Sunday.

"Bob Mueller understands and I understand the specific scope of the investigation, and so no, it's not a fishing expedition," he told host Chris Wallace on "Fox News Sunday."

Rosenstein appointed Mueller to oversee the federal Russia probe, which includes a search for possible collusion with the Trump campaign, back in May after President Trump fired FBI Director James Comey. Because Attorney General Jeff Sessions recused himself from the case, Rosenstein ultimately was given the power to hire, and fire if he has good reason, Mueller.

It has been reported that Mueller's investigation has expanded to look at the finances of Trump and his associates, giving rise to concerns that Mueller could make it the focus of the probe. Rosenstein seemed to dismiss this issue.

Asked if there were no red lines under the terms of his order, Rosenstein said that the investigation is subject to the rules and regulations of the Justice Department.

If there's evidence of a crime that's found within the scope of what they have agreed, then Mueller has free rein, Rosenstein explained. If it's outside the scope of the probe, then "he needs to come to the acting attorney general, at this time me, for permission to expand his investigation," he added. Rosenstein noted this as a precedent, which was also followed by independent counsel Ken Starr during the Clinton years.

Reports have come out over the past couple months about how the probe is investigating Trump for obstruction of justice, and that Mueller has impaneled grand juries, capable of issuing subpoenas.

These reports and more have relied on anonymous sources, against whom Rosenstein cautioned could be unreliable. He dubbed these reports, "speculation in the news media."