The Justice Department's internal watchdog will issue its long-awaited report on the FBI and DOJ's investigation of Hillary Clinton's emails during the 2016 presidential campaign on June 14, Inspector General Michael Horowitz said in a letter to Congress on Thursday.

Horowitz, in a letter to Senate Judiciary Chairman Chuck Grassley, R-Iowa, also said he would accept an invitation to testify before that committee on Monday, June 18.

The DOJ's Office of the Inspector General first announced its investigation of matters related to Clinton's use of a private email server in January 2017, days before President Donald Trump's inauguration.

The watchdog is specifically investigating former FBI Director James Comey's disclosure of public information about the email probe before the November 2016 election, and "whether certain underlying investigative decisions were based on improper considerations."

The full report will be nearly 500 pages long, sources familiar with the matter told The Washington Post. Comey's public comments — such as when he said "no reasonable prosecutor" would bring a case against Clinton for handling classified information on a private server — are expected to be the subject of harsh criticism in the report, according to the Post.

The report is already past due in the eyes of some Republicans.