Democrats’ calls for an independent prosecutor to lead the investigation into Trump campaign contacts with Russia grew louder late last week in the aftermath of the FBI director’s ouster, but the options for an independent probe remain limited, and the only current avenue for such an appointment is in the hands of the Justice Department.

The White House initially said President Donald Trump’s firing of FBI director James Comey has nothing to do with the ongoing counterintelligence probe, but lawmakers from both parties are questioning the timing and some Democrats also are asking whether Trump’s move constitutes interfering in an ongoing investigation.

Now, Trump admits his actions did have to do with the “Russia thing.”

Questions of political interference have dogged the investigation before. In March, Attorney General Jeff Sessions recused himself from anything related to the Russia investigation. On Tuesday, both he and the official currently overseeing the probe, Deputy Attorney General Rod Rosenstein, recommended that Trump fire Comey.

“If there was ever a time when circumstances warranted a special prosecutor, it is right now,” Senate Minority Leader Chuck Schumer said Wednesday.

There is little Congress can do immediately other than apply political pressure to try to force the Trump administration’s Justice Department to name a special counsel. Other alternatives for an independent investigation require congressional approval and, while Democrats are threatening to introduce legislation, a shot at success appears challenging. Senate Majority Leader Mitch McConnell said Wednesday a new probe “could only serve to impede the current work being done.”

Even if a special counsel is appointed, it would be difficult to dispel a perception of impropriety. The person ultimately would answer to the Justice Department and any decisions can be overruled. Plus, in most cases, the special counsel works in secret with no assurance that the probe’s findings will be made public.

In the days before he was fired, Comey told lawmakers he had asked the Justice Department for more resources to pursue the bureau’s investigation, three U.S. officials told The Associated Press. Last Tuesday, the White House released a memo from Rosenstein in which he criticized Comey’s handling of last year’s investigation into Democrat Hillary Clinton’s e-mail practices.

Republican Sen. James Lankford from Oklahoma said Congress and the nation deserve to know why Comey was dismissed.

“I understand the desire to be able to have a special prosecutor,” said Lankford. “Problem is, as we all know, the special prosecutor is selected by the attorney general’s office.”

Democrats also have called for a 9/11-type independent commission and for a select congressional committee to investigate, due to concerns that Republican-led panels may not be motivated to aggressively probe the leader of their party and his campaign. Republicans contend existing bipartisan investigations underway in Congress are sufficient.

But it wasn’t always this way.

Democrats have been drawing parallels between Comey’s firing and President Richard Nixon’s “Saturday Night Massacre” firing of the independent special prosecutor overseeing the Watergate investigation in 1973.

That firing prompted congressional legislation in the late 1970s that authorized the appointment of independent prosecutors by a panel of three judges from the Washington D.C. U.S. Court of Appeals.

The special prosecutor established in that law had more independence than the Justice Department’s current special counsel position. But Congress let the law expire in 1999.

The last high-profile special counsel to be named was in 2003 when the Bush Justice Department turned to Patrick J. Fitzgerald, then the top federal prosecutor in Chicago, to investigate who leaked the identity of Valerie Plame, a covert CIA officer. That appointment was made by Comey, who at the time was deputy attorney general. Comey took the extra step of giving Fitzgerald complete discretion to conduct the investigation, bolstering the special counsel’s independence.

Schumer said Democrats insist a career civil servant at the Justice Department appoint a special counsel to ensure that the American people can have confidence in the criminal justice system. “It should not be a political appointee who makes such a decision,” he said.

House Minority Leader Nancy Pelosi last week threatened to use a rarely successful parliamentary maneuver to force a vote on a bill that would create an independent panel to investigate possible contacts between the Trump campaign and Russian officials. The “fireworks at the Department of Justice demand that we remove the investigation from the Trump-appointed Justice Department leadership,” Pelosi said.

Democratic Sen. Richard Blumenthal said the idea of a special, independent prosecutor is not new. And now is certainly an appropriate time for one.

“I feel strongly that the credibility and independence of this investigation require a special prosecutor,” said Blumenthal, of Connecticut. He also said he’s prepared to introduce legislation to restore the Watergate-era legislation.

If a special counsel is appointed, it would be up to Rosenstein to decide whether to give the prosecutor complete independence. During his Senate confirmation hearing, he was repeatedly pressed by Democrats about whether he would appoint one and would not commit to doing so.

Many Republicans say there is no need for an independent prosecutor or special counsel.

North Carolina Sen. Richard Burr was critical of the president’s decision to fire Comey, but said a special counsel was not necessary. As chairman of the Senate intelligence committee, Burr is leading one of the congressional investigations.

“My committee’s got the jurisdictional responsibility to investigate this,” Burr said. “We are going to do that.”

The committee has sent requests for information and documents to several Trump associates and recently asked the Treasury Department for relevant financial documents. It invited Comey to testify as a private citizen this week.

Associated Press writers Mary Clare Jalonick, Julie Pace and Erica Werner contributed to this report.