Both men had known ties to people suspected of being involved with Russian Intelligence.
The report also rejected claims of political bias from inside the FBI - even as it raised questions about bias from both sides of the aisle.
The report addressed the previously known text messages between FBI lawyer Lisa Page and top counterintelligence official Peter Strzok - but found they did not play any role in the decision to launch the investigation into possible Russian interference or ties to the Trump campaign in 2016.
On the other side, the report also found evidence from some FBI investigators that they favored Mr. Trump - also leaving an electronic paper trail - and in this case, indicating their desire to investigate the Clinton Foundation.
In an odd twist to the public release of the report, Inspector General Michael Horowitz found his conclusions under public attack from the Attorney General of the United States.
"The Inspector General's report now makes clear that the FBI launched an intrusive investigation of a U.S. presidential campaign on the thinnest of suspicions that, in my view, were insufficient to justify the steps taken," Barr said in his own statement, which was at odds with the IG's conclusion.
The skepticism also included a statement from U.S. Attorney John Durham, Barr's handpicked investigator who is doing his own review of the same situation.
For Republicans the report's criticism of possible problems with the FISA process dealing with former Trump campaign adviser Carter Page was the target of most GOP criticism.
In the report, the IG found that there were a number of 'factual misstatements and omissions' in terms of information, which might have undermined what officials thought was an easy decision to sign off on a FISA application for surveillance of Page, who was no stranger to the FBI when it came to Russian intelligence investigations.
"Our review found 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," the IG summary stated.
"We identified multiple instances in which factual assertions relied upon in the first FISA application were inaccurate, incomplete, or unsupported by appropriate documentation, based upon information the FBI had in its possession at the time the application was filed," the report continued.
But the IG did not take any stance on whether the Page FISA requests were improper.