Here's what the new special counsel can — and can't — do with his investigation

• Robert Mueller has been named special counsel investigating the Trump campaign's dealings with Russia.
• Special counsels have very specific sets of powers.
• Other investigative options include special independent commissions and special congressional committees.

The appointment of former FBI Director Robert Mueller as special counsel to investigate the Trump campaign's dealings with Russia was widely praised by both political parties.

But there was less clarity about what exactly he can and can't do.

In his new job, Mueller and his staff can interview witnesses, subpoena documents and, if the evidence merits, work with the bureau to bring criminal charges.

That FBI investigation will continue, even after President Donald Trump fired Director James Comey after reportedly asking him to stand down on his probe into the business dealings of fired national security advisor Michael Flynn. The FBI is also looking into Moscow's interference in the 2016 election.

But some Democrats think the special counsel role doesn't go far enough in insulating Mueller from White House interference.

"Director Mueller will still be in the chain of command under the Trump-appointed leadership of the Justice Department," House Minority Leader Nancy Pelosi, D-Calif., said in a statement. "He cannot take the place of a truly independent, outside commission that is completely free from the Trump administration's meddling."

Four types of investigators:

Special prosecutor: Roots in Watergate

The modern role of special prosecutor dates back to 1973, when President Richard Nixon appointed — and later fired — Archibald Cox to investigate the Watergate break-in.

To create more independence for the role, Congress in 1978 passed the Ethics in Government Act, which allowed the attorney general to ask a three-judge panel to authorize a special prosecutor if deemed necessary. That law was invoked during the Clinton administration, when Kenneth Starr conducted a lengthy investigation covering various lines of inquiry.

Special counsel: How Mueller's role works now

In 1999, however, Congress let the special prosecutor law expire and set up new rules for the attorney general to appoint a special counsel.

Under the current law, the special counsel's focus is somewhat more limited, with the scope of the investigation set by the attorney general and confined to investigating criminal matters.

Saul Loeb | AFP | Getty Images

Robert Mueller

The job is the equivalent of a U.S. attorney, but is not subject to day-to-day supervision by the Justice Department. The special counsel has to be named from outside government and has broad leeway in staffing any investigation.

As special counsel, Mueller technically works for the DOJ, so the attorney general has the authority to fire him but only for "misconduct, dereliction of duty, incapacity, conflict of interest, or for other good cause, including violation of Departmental policies."

But because Attorney General Jeff Sessions has recused himself from the case, that authority falls to Deputy Attorney General Rod Rosenstein, who appointed Mueller.

Independent commission: Reports, but no prosecutions

Congress has the authority to create an independent commission to conduct an investigation, much as it did after the assassination of President John Kennedy and the 9/11 attacks.

That would require a special act of Congress, the appointment of commissioners from both parties and the formation of an independent staff of investigators. The commission could be given subpoena powers, as Congress did for the 9/11 Commission.

But the result of such a commission's investigation is only a report; it would not have the power to press charges or seek civil penalties.

Given the current GOP control of both chambers of Congress, the creation of an independent commission is unlikely.

Special congressional committee: One too many now?

Congress can set up committees for specific investigations, but at this point there are already separate congressional probes underway, the primary ones being handled by the Senate Intelligence Committee and House Intelligence Committee.

Other investigations are also underway by the Senate Judiciary and House Oversight committees.

Some Republicans have argued that appointing a special prosecutor would complicate those ongoing investigations, especially when it comes to issuing subpoenas or granting immunity to witnesses.

But, aside from playing out in public, the ongoing congressional investigations serve a different purpose, according to Sen. Susan Collins, R-Maine.

"We have two very different missions," she told MSNBC. "The special counsel will focus on whether or not criminal charges are appropriate. We're focused on counterintelligence policy. And we can impose sanctions on Russia. The special counsel can't impose sanctions on Russia. So we still have an extremely important role to play."

Correction: The Ethics in Government Act was passed by Congress in 1978. An earlier version misstated the year.