The 500-page report examines actions by top FBI and justice department officials before the 2016 election.
The inspector general's report will be released in full later during the day.
The report found while Comey's actions were not the result of political bias, "by departing so clearly and dramatically from FBI and department norms, the decisions negatively impacted the perception of the FBI and the department as fair administrators of justice", according to Bloomberg News.
In announcing the review in January 2017, Horowitz said that there was overwhelming demand from lawmakers, members of the public, and "various organisations" to investigate claims of unprofessional behaviour on the part of the justice department and FBI employees.
The report focuses on Comey's decision in July 2016, then FBI director, to hold a news conference announcing that Hillary Clinton would not be criminally charged for using a private email server for classified information during her time as US Secretary of State.
Eleven days before the presidential election, Mr Comey announced that the bureau was investigating new emails related to the Clinton email probe.
Mrs Clinton and her aides have blamed Mr Comey for her election loss to Mr Trump while Democrats pointed out he that remained silent at the time about an investigation into possible collusion between Russia and the Trump campaign.
Mr Trump has repeatedly denied any collusion with the Kremlin, calling Russia investigation a "witch hunt".
The report criticises Mr Comey and other senior officials for decisions made in the lead-up to the US election in 2016.
But the report also touched on text messages exchanged between two FBI officials who worked on Special Counsel Mueller's investigation.
Republicans have seized on exchanges between Peter Strzok, a senior counter-intelligence agent, and Lisa Page, an FBI lawyer who also temporarily worked on the Mueller investigation and with whom Mr Strzok was having an extramarital affair, to argue that the agency was biased against Mr Trump.
When Ms Page asked if Mr Trump would become president, Mr Strzok responded: ""No. No he won't. We'll stop it," according to a copy of the report, obtained by the Washington Post.
Mr Trump has also criticised Mr Comey for his handling of the Clinton email investigation.
"After years of Comey, with the phony and dishonest Clinton investigation (and more), running the FBI, its reputation is in Tatters - worst in History! But fear not, we will bring it back to greatness," Mr Trump tweeted in December.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)