Although it is not unlawful for the FBI to hire informants, it fell short of adhering to policy while looking into Trump's aides, as per the IG report
In July 2016, the FBI launched a counterintelligence investigation “Crossfire Hurricane” to gauge the scope and nature of Russia’s efforts at influencing the U.S. presidential elections in 2016. The FBI tasked Stephen Halper, a professor emeritus at the University of Cambridge, to contact three individuals affiliated with the Trump presidential campaign — foreign policy advisers Carter Page and George Papadopoulos, and campaign co-chair Sam Clovis. Halper had a long history of involvement in the U.S. government and on political campaigns, including stints in the administrations of Republicans Richard Nixon, Gerald Ford, and Ronald Reagan, as per BBC.
In a report published on May 24, 2018, the BBC stated that Halper identified himself as an informant who worked with the FBI to gather information on at least three members of the Trump campaign team it suspected could have ties to the Russian government.
Page was reported to have attracted the FBI's attention because of previous connections to Russian intelligence agents, as well as a Moscow trip while working for the Trump campaign. Papadopoulos was brought under the ambit of the investigation after he revealed to an Australian diplomat that the Russian government had political dirt on Hillary Clinton. Halper was reported to have met all three as the FBI’s informant for gathering information on their roles in furthering Russia’s interference in the elections. FBI’s deployment of Halper as an informant led to Trump supporters questioning the legitimacy of the investigation. The FBI website says that U.S. courts have recognized that the use of informants is "lawful and often essential to the effectiveness of properly authorized law enforcement investigations." The bureau also notes that "special care is taken to evaluate carefully and closely supervise their use, so the rights of individuals under investigation are not infringed."
In May 2017, U.S. Deputy Attorney General appointed former FBI boss Robert Mueller as special counsel to investigate Russian interference in the 2016 election and any alleged collusion with the Trump campaign. While the 448-page report did not conclude that there was a criminal conspiracy between Moscow and the Trump campaign to influence the 2016 U.S. presidential election, it did detail 10 instances where Trump possibly obstructed justice. The investigation produced 37 indictments; seven guilty pleas or convictions; and compelling evidence that Trump obstructed justice on multiple occasions. No members of the Trump family were charged. Mueller and his team concluded that they were unable to charge the president with a crime, but could not exonerate him either. In July 2019, Mueller reaffirmed before Congress that his report did not exonerate the president.
On December 9, 2019, Justice Department Inspector General Michael Horowitz’s report on the origins of the Russia investigations was published. It stated, “We found no evidence that the FBI attempted to place any CHSs (Confidential Human Sources) within the Trump campaign, recruit members of the Trump campaign as CHSs, or task CHSs to report on the Trump campaign.”
However, the report also found 17 errors or omissions made by the FBI as it sought and received approvals to conduct surveillance on Page. Those applications were filed to the secret court created by the Foreign Intelligence Surveillance Act, or FISA, to oversee intelligence and terrorism cases. With regards to Page, the Mueller report concluded that “the investigation did not establish that Page coordinated with the Russian government in its efforts to interfere with the 2016 presidential election.”The IG also observed that “FBI personnel fell far short of the requirement in FBI policy that they ensure that all factual statements in a FISA application are ‘scrupulously accurate." Besides, the FBI had also gathered information suggesting that reports by former British intelligence officer Christopher Steele, which were a key element justifying the surveillance application, were not as reliable as officials had described to the court that ultimately approved the warrant to wiretap Page multiple times, reported The Washington Post.
While the report by the Justice Department watchdog made it abundantly clear that the FBI had mishandled the investigation especially with regard to monitoring Page, it also asserted that the FBI was justified in opening its 2016 investigation into the Trump campaign and that there was no indication of political bias in the probe. This contradicted the narrative that Trump has frequently pushed that he was the victim of a political conspiracy. Trump has even used the word ‘coup’ to describe efforts made by law enforcement to probe Russia’s role in the 2016 U.S. elections. He used similar tactics in March 2017, when he accused former President Barack Obama of wiretapping Trump Tower during the 2016 election campaign but the Department of Justice ruled that no evidence of wiretapping was found.
In Jan 2020, the Justice Department concluded in a declassified judicial order that two of the four court orders allowing the FBI to conduct secret national security surveillance on Page were not valid because the government made "material misstatements" in obtaining them. In August 2020, former FBI lawyer Kevin Clinesmith pleaded guilty to altering an email that one of his colleagues relied on as he sought approval to surveil Page during the FBI’s investigation. The email from another FBI official was changed by Clinesmith to suggest that Page, had never been a CIA "source" - whereas Page was in fact a CIA informant.
American journalist Glenn Greenwald wrote in The Intercept, “In sum, the IG Report documents multiple instances in which the FBI – in order to convince a FISA court to allow it spy on former Trump campaign operative Carter Page during the 2016 election – manipulated documents, concealed crucial exonerating evidence, and touted what it knew were unreliable if not outright false claims.”