The struggle to uncover the FBI's conduct in the Trump-Russia probe has made some congressional investigators deeply suspicious of the bureau. But what do those investigators think the FBI actually did in its investigation of the Trump campaign, Russia, and the 2016 election?

First, they're convinced the FBI has something to hide. In the last 12 months, the bureau has, at various times, ignored, slow-walked, resisted, and downright stonewalled congressional requests, not to mention subpoenas, for information on the Trump-Russia investigation.

Each time the bureau hunkered down, suspicion grew on Capitol Hill. The FBI seemed particularly reluctant to reveal to Congress not what Russians did, or what people in the Trump circle did, but what the bureau itself did.

When did the investigation start? How did it start? What measures did the FBI, and its lawyers, and its informants employ? Getting facts out of the FBI has been a long and arduous task.

First to cause serious suspicion was the Trump dossier. Eyebrows were raised when investigators learned that the FBI, at the height of the 2016 presidential campaign, offered to hire a former British spy who was collecting allegations about Trump and Russia.

House Intelligence Committee Chairman Rep. Devin Nunes, R-Calif., pushed for information. Among other things, he learned that the former British spy, well-connected with the FBI, was paid by the Hillary Clinton campaign and the Democratic National Committee. That apparently did not matter to the bureau.

Then Nunes and others wondered: What did the bureau do with the sensational allegations in the dossier? That gave birth to the so-called "FISA abuse" investigation, when Republicans looked into whether the FBI used unverified allegations from the Trump dossier in proceedings before the Foreign Intelligence Surveillance Act court. It turned out the FBI did just that, to win a wiretap on Carter Page, who for a short time was a volunteer on candidate Trump's foreign policy advisory board.

Congressional investigators came away with the impression that the FBI was hiding something. It was.

Now, Congress is trying to get information about the informant(s) the FBI used in the Trump-Russia probe, and precisely what those informants did.

As part of that line of inquiry, investigators have discovered a number of times in which Trump figures were approached, sometimes by people with FBI connections, with offers of derogatory information on Clinton. Each incident was before the FBI says it began the Trump-Russia investigation, code name Crossfire Hurricane, on July 31, 2016.

Starting in late May or early June 2016, a Cambridge University professor named Stefan Halper, who was a longtime FBI informant, contacted Page, and also Trump campaign official Sam Clovis, and finally Trump volunteer adviser George Papadopoulos, seeking contacts and information on the campaign. The Daily Caller's Chuck Ross has reported that Halper "randomly asked Papadopoulos whether he knew about Democratic National Committee emails that had been hacked and leaked by Russians."

In early June 2016, a Russian lawyer who was working closely with the opposition research firm Fusion GPS, which had commissioned the Trump dossier, asked for and received a meeting with top Trump campaign officials, including Donald Trump Jr., by promising dirt on Clinton. The meeting came to nothing.

In late May 2016, a Russian who had apparently been an FBI informant for years contacted an associate of Trump campaign official Michael Caputo, and later met with Trump figure Roger Stone, reportedly offering dirt on Hillary Clinton. Stone told the Washington Post the Russian asked for $2 million, and the meeting went nowhere. The Russian said he was not working for the FBI when he met Stone.

Stone has on a variety of occasions denied he met with any Russians during the campaign — so take that into account when considering his credibility. The problem for congressional investigators is that the probe has become a two-front battle: dealing with the untrustworthiness of some of the figures in the investigation while also fighting the FBI to learn the basic facts of what happened.

Lawmakers would not be shocked that Roger Stone might lie to them. But they expect the FBI to be open and transparent with constitutionally-empowered oversight committees.

The bottom line is that some Republicans are wondering whether in the above instances, and perhaps others, someone actively tried to frame, or entrap, or set up, Trump figures. And those Republicans wonder whether the FBI knew about it or played some sort of role in it.

In short, there is suspicion that the FBI might have abused its tremendous powers in a highly politicized investigation undertaken in the middle of a presidential campaign.

The suspicions are behind the House move to force the FBI to give up information. Last Friday evening, top House lawmakers, including Speaker Paul Ryan, R-Wis., met with bureau and Justice Department officials to demand compliance with House subpoenas. They gave the FBI a tight deadline to produce the subpoenaed information or face serious retaliation, like contempt proceedings, from the House.

Some Republicans believe the FBI will, finally, comply. Maybe that will happen, and maybe it won't. But the only thing that can reduce suspicion in the current atmosphere is more openness.