By Andy Sullivan and Mark Hosenball

WASHINGTON (Reuters) – The FBI has not done enough to fight homegrown extremist threats and has yet to figure out how to determine whether people it investigates who have mental health issues pose an actual threat to national security, the U.S. Justice Department’s internal watchdog said on Wednesday.

Justice Department Inspector General Michael Horowitz also found that the FBI did not follow up on some cases that had been flagged as potential threats.

The report found shortcomings in the FBI’s efforts to prevent mass attacks by U.S. residents who have been inspired by international militant groups like Islamic State and al Qaeda, which the agency says its highest counterterrorism priority.

The FBI did not immediately respond to a request for comment.

So-called homegrown violent extremists have carried out more than 20 attacks in the United States since Sept. 11, 2001 – some perpetrated by people the FBI had already investigated.

Horowitz found that the FBI conducted reviews after those attacks to find out what it had missed, but didn’t make sure that agents followed through with its proposed improvements.

The FBI concluded in 2017 that it should have conducted about 6 percent of its counterterrorism assessments more thoroughly. But the agency did not re-examine nearly half of those cases for 18 months, Horowitz wrote.

(Reporting by Andy Sullivan and Mark Hosenball; Editing by Chizu Nomiyama and Jonathan Oatis)