Russian President Vladimir Putin. Adam Berry/Getty Images A high-level official at the Department of Justice tasked with investigating Russia's interference in the 2016 election has announced that she will leave the DOJ in May, leaving a key position in the department's National Security Division unfilled as President Donald Trump's political appointees await confirmation in the Senate.

Mary McCord, the acting assistant attorney general of the division, did not provide a reason when she told her staff that she would be leaving in May, according to NPR. She said "the time is now right for me to pursue new career opportunities."

McCord's departure has raised questions about the future of the Trump-Russia investigation, which will be in the hands of Trump's nominee for deputy attorney general, Rod Rosenstein, if and when he is confirmed. Attorney General Jeff Sessions recused himself from Trump-campaign-related investigations last month amid revelations that he failed to disclose two meetings he had with Russia's ambassador to the US, Sergey Kislyak, in 2016.

"This just highlights how important it is that the Russia investigation be handled by an independent prosecutor," said Matt Miller, a DOJ spokesman under the Obama administration. "Once Rod Rosenstein is confirmed, the investigation will be in the hands of someone who interacts with people in the White House on a daily basis, and that's just not tenable. It's even harder with the career official who has been handling it leaving the department."

When asked during his confirmation hearing last month whether he would select a special prosecutor to oversee the Russia investigation, Rosenstein replied that he was "not in a position to answer."

US President Donald Trump with Secretary of State Rex Tillerson during a bilateral meeting with Chinese President Xi Jinping at Trump's Mar-a-Lago estate in Palm Beach, Florida. Thomson Reuters

"I view it as an issue of principle that as a nominee for deputy attorney general, I should not be promising to take action on a particular case," Rosenstein said. "I believe that if I were to do this in this case, some future deputy attorney general nominee would be … asked to make a similar commitment, and they'd say, 'Rosenstein did it — why won't you?'"

Rosenstein added that he had not yet read the intelligence community's report on Russia's election interference and knew only what he had seen in media reports.

Democrats concerned about Rosenstein's ability to lead an unbiased investigation into the Trump campaign's ties with Moscow vowed to block his confirmation if he did not commit to appointing a special counsel.

"I'll use every possible tool to block DOJ Deputy AG nominee [Rosenstein] unless he commits to appoint independent special prosecutor," Democratic Sen. Richard Blumenthal tweeted at the time.

(As lawyer Paul Rosenzweig has written, however, the correct term is "special counsel," not "special prosecutor.")

In any case, with McCord gone and Rosenstein set to take over, calls are growing louder for an independent counsel to oversee the criminal investigation into Russia's election interference and the Trump campaign's possible role in it. (The FBI's intelligence activities are overseen by the Director of National Intelligence.)

"The final step would be for the Attorney General to select to lead the inquiry a prominent Republican — perhaps one who is not affiliated with the #NeverTrump wing of the party," Rosenzweig wrote on Lawfare. "We might also consider convening a special grand jury, which could provide the American public with a report on its investigation."

"The ideal preference, of course, would be for this all to run in parallel with a Congressional investigation," he added, "but that may be too much to ask for."

The congressional intelligence committees' respective investigations into Russian meddling and who knew about it have been slow-going. The House Intelligence Committee's chairman, Rep. Devin Nunes, stepped aside earlier this month amid questions about his ability to lead an independent investigation into Trump's Russia ties. Republican Rep. Mike Conaway is set to take over for Nunes, but he has cast doubt on the intelligence community's conclusion that Russia interfered in the election to help Trump win.

An open hearing featuring testimony from former acting Attorney General Sally Yates, former Director of National Intelligence James Clapper, and former CIA Director John Brennan was scrapped by Nunes late last month and hasn't been rescheduled.

Yates warned the administration in January that Trump's former national security adviser, Michael Flynn, could be subject to Russian blackmail, The Washington Post reported in February. Brennan, for his part, reportedly established a counterintelligence task force last summer to examine reports of improper contact between the Trump campaign and the Kremlin handed to the CIA by foreign intelligence agencies.

The Senate Intelligence Committee, meanwhile, held its first open hearing on Russian meddling late last month, and it has not yet scheduled any more open hearings related to its Russia investigation.