In late July, the Justice Department refused a request from the Senate Judiciary Committee — a bipartisan, joint request from Chairman Charles Grassley and Ranking Democrat Dianne Feinstein — to make two top FBI officials available for an interview in the committee's investigation of the Trump dossier and other matters related to the Trump-Russia affair. Citing the Mueller special prosecutor investigation, Justice stated "confidentiality" and the "sensitivity of information relating to pending matters" made it impossible for the two officials, Carl Ghattas and James Rybicki, to talk to the Senate committee that oversees the FBI.

Grassley and Feinstein are still trying — they sent another, more strongly worded, request last Friday. Their efforts show the importance of the FBI in Congress' quest to learn more about the "salacious and unverified" dossier (the words of former FBI Director James Comey), and could signal the FBI will play a key role in Congress' dossier investigation as it plays out in coming months.

Just last week, Glenn Simpson, head of Fusion GPS, the opposition research firm that handled the Trump dossier, refused to tell Grassley's and Feinstein's investigators who funded the effort. But there are other ways to get at the story — and the FBI is the number-one possibility.

That's because the FBI played a role in the case as it happened. Sometime in the process of collecting anti-Trump allegations from paid, Kremlin-linked Russian informants, Christopher Steele — the former British spy hired by Fusion to dig dirt in Russia — decided to take his information to the FBI. That appears to have been in the fall of 2016, at the height of the presidential campaign.

The FBI took the dossier seriously, in part because agents had dealt with Steele before in 2010 in the investigation into FIFA, the world soccer organization. So in October, Steele "reached an agreement with the FBI a few weeks before the election for the bureau to pay him to continue his work," according to a Washington Post report.

It was a mind-boggling development: Federal law enforcement agreeing to fund an ongoing opposition research project being conducted on behalf of one of the candidates in a presidential election. In the end, the FBI reportedly did not pay Steele, possibly because of publicity concerns.

One person whose mind was boggled by the news was Grassley. The Judiciary Committee chairman thought through the implications of the FBI adopting the Fusion-Steele dossier project, and on March 6 sent then-FBI Director James Comey a letter.

"The idea that the FBI and associates of the Clinton campaign would pay Mr. Steele to investigate the Republican nominee for president in the run-up to the election raises further questions about the FBI's independence from politics, as well as the Obama administration's use of law enforcement and intelligence agencies for political ends," Grassley wrote.

Grassley demanded all FBI materials relating to the dossier, plus information on what, if any, actions the bureau undertook to try to verify its contents, and how the bureau used the information. Grassley was particularly concerned about whether the FBI ever presented material from the dossier — unverified, from paid informants — to a court as a basis for obtaining a warrant in the Russia investigation. To do so would amount to using false pretenses to seek court permission to put someone under surveillance.

"Has the FBI relied on or otherwise referenced the memos or any information in the memos in seeking a FISA warrant, other search warrant, or any other judicial process?" Grassley wrote to Comey. "Did the FBI rely on or otherwise reference the memos in relation to any National Security Letters?"

On March 15, Comey personally briefed Grassley and Feinstein on the Russia investigation. Whatever Comey said, Grassley was not fully satisfied. Nor was he satisfied in the weeks that followed. On April 28, Grassley wrote to Comey, "The FBI has failed to provide documents requested in the March 6 letter or to answer the vast majority of its questions."

As the days dragged on, Grassley clearly felt the FBI was not telling the truth, or certainly not the whole truth, about the bureau, Christopher Steele, and the dossier.

"There appear to be material inconsistencies between the description of the FBI's relationship with Mr. Steele that you did provide in your briefing and information contained in Justice Department documents made available to the Committee only after the briefing," Grassley wrote to Comey. "Whether those inconsistencies were honest mistakes or an attempt to downplay the actual extent of the FBI's relationship with Mr. Steele, it is essential that the FBI fully answer all of the questions from the March 6 letter and provide all the requested documents in order to resolve these and related issues."

A few days later in an oversight hearing on May 3, Comey refused to answer again — at least in public — when Grassley asked whether Steele paid his sources. But at Grassley's prodding, Comey did concede that it was a "vital" issue.

"Was the FBI aware that Mr. Steele reportedly paid his sources who in turn paid their sub-sources to make the claim in the dossier?" Grassley asked.

"Same answer, sir," Comey said, referring to earlier refusals to answer.

"Here's one you ought to be able to answer," Grassley said. "Is it vital to know whether or not sources have been paid in order to evaluate their credibility and if they have been paid doesn't that information need to be disclosed if you're relying on that information in seeking approval for investigative authority?"

"I think in general yes," Comey answered. "I think it is vital to know."

Still, the committee got little information out of the FBI. Grassley and others remained concerned — and suspicious — on the question of whether the FBI used paid, unverified information from the dossier in any attempts to get surveillance warrants.

On June 27, Grassley and Sen. Lindsey Graham, chair of the crime and terrorism subcommittee, tried another tack. They wrote both the FBI and the Justice Department to ask for "all proposed and final applications for surveillance warrants that the FBI reportedly sought from the Foreign Intelligence Surveillance Court in the investigation into Russian interference in the 2016 elections."

"Media reports and the [surveillance court's] 2016 annual report provide reason to believe that, in the course of these investigations, the FBI and Justice Department may have submitted FISA applications that the court preliminarily evaluated and stated it would reject, which the FBI and Justice Department then modified and resubmitted," Grassley and Graham wrote. In addition, the two noted, the often rubber-stamp surveillance court "denied nine applications or certifications [in 2016], and denied in part or modified 365 orders."

Grassley and Graham suspected something was up.

On July 11 came the request — this time not only from Grassley but from Democrat Feinstein, too — to question the FBI's Ghattas and Rybicki. On July 27 came the Justice Department's refusal. It's likely Ghattas and Rybicki know the dossier story from the FBI's perspective. Who decided to pay Christopher Steele for the information? Did the payments happen or not? Either way, why? And what about the dossier information and the warrants?

So there are, in effect, two Senate investigations into the dossier now — one focused on Fusion GPS and the people associated with it, and the other focused on the FBI. And don't forget the House Intelligence Committee, which is also interested in the dossier.

The dossier remains critical in the Trump-Russia investigation for several reasons. It contains a number of sensational allegations on the question of whether the Trump campaign colluded with Russians. It raises questions about Clinton supporters indirectly paying Kremlin-linked Russians for dirt on the Republican presidential candidate, now the president. And it played a role in the rapid deterioration of relations between Trump and Comey.

For all those reasons and more, Senate investigators want to learn the full story of the dossier. And if they can't find out one way, they'll try another.