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Whether it was the Sept. 11 terrorist attacks or the financial downturn of the last decade, Congress has in the past launched independent investigations to nail down answers.

Some members of Congress are convinced now is the time for another full-scale inquiry over an issue they say has put America's national security at risk: Russia's alleged interference in the 2016 election.

The calls for an independent review are growing louder after the chairman of the House Intelligence Committee, GOP Rep. Devin Nunes, admitted to meeting with a secret source at the White House to look at intelligence reports without notifying fellow committee members. Nunes was also on Donald Trump's transition team.

Congress, as well as the FBI, is investigating whether Trump aides or his campaign team had any illegal contact with the Russians last year.

Related: Nunes Had Secret White House Meeting Before Trump Monitoring Claim

With so much on the line, Democratic committee members say Nunes' White House connection is eroding credibility and Democrats in the House and Senate along with a handful of Republicans are calling for an independent commission. But getting one off the ground won't be easy — particularly in such a partisan political climate, observers say. Here's why:

An independent commission needs funding.

Investigative commissions are fairly rare, with just seven formed between 1989 and 2016. One major question is whether Congress would be willing to shell out money for an investigation.

Paying for a full-time staff to conduct interviews and hearings, sometimes over years, can run from several thousands of dollars to over $10 million, according to a paper released in January by the nonpartisan Congressional Research Service.

The investigation into the intelligence failures of 9/11 — commonly known as the 9/11 Commission — employed 80 people and was initially approved for $3 million. But the panel asked for more money in order to complete the work, and after butting heads with the George W. Bush White House, an additional $9 million was allocated.

Since the money comes from the budget, it requires an approval from Congress through an appropriations bill.

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Lee Hamilton, the vice chair of the 9/11 Commission, said "sustained funding" would be crucial, although fiscally conservative members of Congress could decide to block any spending toward an investigation. Locating private funding could be another option, he added, although that could be difficult to secure.

The GOP, the Democrats and the White House would all have to be on board.

Getting an appropriations bill through a Republican-controlled Congress is one thing, but getting the White House to sign off is another, Hamilton said.

"Trump could veto it," he said, adding that without the funding, an independent investigation would never take off.

That could change, however, if the public pressure for an investigation becomes overwhelming and the White House and Congress felt compelled to act, Hamilton said.

George W. Bush was "deeply suspicious" of the 9/11 Commission, Hamilton said, but it took his administration's willingness to cooperate in the investigation to be able to put out a thorough report.

Even if Democrats somehow managed to arrange a privately funded investigation, its findings could be called into question by Republicans.

"You certainly can't do it without them," said Hamilton, now the director of the Center on Congress at Indiana University.

Congress would have to agree on the scope of the commission.

A number of details must be ironed out by lawmakers. Those include who would sit on the committee (generally a 10-member bipartisan panel), how far-reaching the investigation would be and who would be called to testify.

Any wrangling over those key issues could threaten to derail a commission.

Even something like a deadline for final recommendations could prove problematic, Hamilton said. The work of the 9/11 Commission took almost two years from when it was set up to when it was officially closed.

"Working against a deadline is very undesirable," Hamilton said. "You want to have plenty of time to conduct the investigation."

Even if a commission is formed, that wouldn't stop other ongoing investigations.

Both the House and Senate intelligence committees are pursuing their own inquiries, while FBI Director James Comey has his own probe into whether there was any collusion between the Trump campaign and Russia.

Some members of Congress say they're comfortable with Congress or the FBI determining the truth, and think an independent commission would be a waste of resources.

"I heard my friend from California (Democratic Rep. Adam Schiff) mention an independent commission," Rep. Trey Gowdy, R-S.C., said Sunday on CBS' "Face the Nation." "Thank goodness we have one. It is called the FBI."

Related: Schiff Calls on GOP Intel Chair Nunes to Recuse Himself From Russia Probe

Nunes, meanwhile, has indicated that he doesn't plan on recusing himself as head of the House Intelligence Committee. He could be forcibly removed and replaced by the next senior Republican on the committee if House Speaker Paul Ryan wants him out, said Beth Rosenson, a University of Florida political science professor.

But Nunes doesn't appear to be going anywhere. When asked whether Nunes should step down, Ryan told reporters Tuesday, "No."