Michael Zeldin, a CNN legal analyst, has served as a federal prosecutor in the Criminal Division of the Department of Justice and was a special counsel to then-Assistant Attorney General Robert Mueller. The opinions expressed in this commentary are his own.

(CNN) In recent weeks, there has been a lot of discussion about the scope of Special Counsel Robert Mueller's mandate and whether he has crossed some imaginary red line that President Donald Trump appears to have drawn.

The New York Times recently asked President Trump, "if Mueller was looking at your finances and your family finances unrelated to Russia, is that a red line?" He responded , in part: "I would say yeah. I would say yes."

Since then, legal and political commentators have debated whether there should be a red line and, if so, what should happen if Mueller were to cross it.

To understand the discussion, one must be familiar with the mandate itself and the provisions of the Code of Federal Regulations, 28 C.F.R. Part 600, which govern the conduct of the Special Counsel.

On May 17, 2017, "to ensure a full and thorough investigation of the Russian government's efforts to interfere with the 2016 presidential election," Acting Attorney General Rod J. Rosenstein appointed Mueller to serve as special counsel for the Justice Department.

Rosenstein's order authorizes the special counsel "to conduct the investigation confirmed by then-FBI Director James B. Comey in testimony before that House Permanent Select Committee on Intelligence on March 20, 2017, including:

i) any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump; and

ii) any matters that arose or may arise directly from the investigation, and

iii) any other matters with the scope of 28 C.F.R. Section 600.4(a)."

Section 600.4(a) grants the special counsel the authority to "investigate and prosecute federal crimes committed in the course of, and with the intent to interfere with, the Special Counsel's investigation, such as perjury, obstruction of justice, destruction of evidence and intimidation of witnesses."

During his March 20, 2017 testimony before the House Permanent Select Committee on Investigations referenced in Rosenstein's order, Comey testified that: "I have been authorized by the Department of Justice to confirm that the FBI, as part of our counterintelligence mission, is investigating the Russian government's efforts to interfere in the 2016 presidential election, and that includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia's efforts. As with any counterintelligence investigation, this will also include an assessment of whether any crimes were committed."

Broad scope

Taken all together, Mueller's investigative mandate is broad.

Its scope seems clear and includes investigating:

(1) Russian efforts to interfere in the 2016 presidential election;

(2) the nature of any links between the Russian government and any and all individuals associated with the Trump presidential campaign;

(3) the nature of any coordination between the Russian government and any and all individuals associated with the Trump presidential campaign;

(4) any matters that have arisen out of the Russian election interference investigation;

(5) any matters that might arise directly from the Russian election interference investigation; and

(6) any federal crimes committed in the course of or with the intent to interfere with the investigation.

The core question is not a simple one: whether there were any links and/or coordination between the Trump campaign and representatives of the Russian government ("collusion" in the term popularized by the media; "conspiracy" in legal terms). This is because direct evidence of any collusion or conspiracy may be difficult to come by.

Not Perry Mason

It is unlikely that a signed agreement between the parties would exist. It is not probable that written documents expressly evidence a quid pro quo. It would be surprising if there were any Perry Mason-style confessions by witnesses appearing before the grand jury.

To the extent that any evidence of a conspiracy were to exist and the conspiracy could be proven, it is likely that most of the evidence would be circumstantial -- that is, it may only consist of verbal understandings and tacit agreements through financial and other intermediaries and close business associates.

Witness testimony, especially from individuals with knowledge of the core questions, will be critical. Financial relationships and dealings, even those remote in place and time, may be important to shed light on motive, intent or susceptibility to compromise.

It can be challenging, however, for a prosecutor to succeed in persuading a witness to cooperate, particularly in a case like this. What is often required is leverage and incentive. Leverage and incentive are most readily gained when a prosecutor can establish that a witness participated in some other readily provable crime.

Generally, the crime does not have to be directly connected or related to the core matter under investigation, but given Mueller's mandate and the limitations imposed by Section 600.4(a), it would seem that the offense would need to be related in some way to the Russian investigation.

What prosecutors do

At the early stages of an investigation, particularly one like Muller's that is complex, it is often impossible to determine what is or is not related to the core question under investigation. Accordingly, it is a sound prosecutorial practice to pursue all leads, especially those that involve financial or other business dealings that may shed light on the parties' relationships. Thus, predetermining relevance at this stage of the investigation could be a mistake.

As Mueller's investigation matures, it may come to pass that some of the leads investigated (including possibly Trump's and/or his family's financial dealings) will prove to have merit, but that they may be beyond the scope of Mueller's original mandate. If so, what Mueller and Rosenstein will have to consider is how best to resolve these issues. Section 600.4 provides a mechanism.

It provides: "If in the course of his or her investigation the special counsel concludes that additional jurisdiction beyond that specified in his or her original jurisdiction is necessary in order to fully investigate and resolve the matters assigned, or to investigate new matters that come to light in the course of his or her investigation, he or she shall consult with the Attorney General, who will determine whether to include the additional matters within the special counsel's jurisdiction or assign them elsewhere."

As such, Rosenstein, acting in consultation with Mueller, must reach a decision as to whether and in what circumstances it would be in the best interests of justice to expand the original mandate to allow the special counsel to investigate and resolve fully these matters and/or to investigate any other new matters that may have come to light in the course of the investigation.

If a decision were made not to expand the mandate, the activities that are not within the original mandate could be referred either to the Department of Justice or to the US Attorney's office with jurisdiction over the matter for completion.

Those who worry about Mueller having an unlimited budget to conduct a "fishing expedition" should be reminded that Section 600.8(a) requires the special counsel to report to Acting Attorney General Rosenstein on the status of the investigation and to seek approval of an annual budget. The acting attorney general will determine annually whether the investigation should continue and, if so, he will establish the budget for the year. This serves as a restraint on unchecked spending and an overly broad investigation.

Too early to cut off any leads

In an investigation such as this, with so many aspects, allegations, and participants spread across the globe, it is much too early to cut off any investigative leads.

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The financial dealings of the Trump businesses and those of his family and business associates, whether directly related or not, may provide an understanding that is relevant to the heart of the investigation. If they do, they are, de facto, not unrelated.

If the financial dealings prove to be unrelated, the regulations offer two possible solutions -- refer that aspect of the investigation to the Department of Justice to handle, or expand the mandate to cover the resolution of the matter. In either case, justice will be served.