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– The Justice Department inspector general on Wednesday chided the FBI for failing to fully address weaknesses in how it assesses possible homegrown terrorists — an area in which officials have been working to improve after several attacks by suspects previously known to law enforcement.

In a 41-page report, Inspector General Michael Horowitz wrote that while the bureau seemed to recognize its failings, it had "not taken sufficient action" to fix the problems.

For example, Horowitz wrote, the FBI issued a new policy in September 2015 requiring field offices to conduct specific database checks and interview subjects of counterterrorism assessments. But a review found the field offices did not always comply.

In 2017, Horowitz wrote, the FBI found some potential terrorist threats that might not have been adequately examined from 2014 to 2016. But officials did not take adequate action on nearly 40% of those cases for 18 months, he wrote.

"As a result, potential terrorist threats were not mitigated for more than 1 year," Horowitz wrote.

How the bureau assesses homegrown violent extremists — those in the United States inspired to attack by terrorist organizations abroad — is extremely consequential. Horowitz noted that in the more than 20 homegrown terrorist attacks in the United States since Sept. 11, 2001, several suspects had been previously assessed by the bureau as a potential threat but not fully investigated.

Among them were Nidal Hasan, who shot and killed 13 people at Fort Hood, Texas, in 2009; Tamerlan Tsarnaev, who, along with his brother, Dzhokhar Tsarnaev, killed three people and injured hundreds in the 2013 Boston Marathon bombing; and Omar Mateen, who shot and killed 49 people in Orlando's Pulse nightclub in 2016.

For each man, Horowitz wrote, the bureau opened an assessment but ultimately decided the suspect did not pose a threat to national security. An assessment is a precursor to a preliminary or full investigation, in which the bureau can take more aggressive steps.

Horowitz wrote that an internal FBI document said the bureau had a "fundamentally incomplete understanding" of the threat posed by such extremists and that deciphering whether suspects are merely consuming terrorist propaganda or planning an attack "is extremely complex."

In a letter included in Horowitz's report, Suzanne Turner, chief of the FBI's External Audit and Compliance Section, wrote that the bureau has "made changes to implement best practices and make recommended changes."

Officials issued a policy in 2015 about what information should be collected about possible threats, then conducted further study and issued more guidance the following year. But Horowitz wrote that officials did not make sure the directives were implemented, and "FBI field offices continued to conduct assessments that did not meet FBI requirements or standards."

Horowitz also criticized the bureau for not developing strategies to address "the challenges associated with the potential cross over between terrorist threats and other categories of threats, for example, those posed by individuals with mental health issues and criminal threats to life."

Horowitz noted that the bureau had shown a commitment to combating homegrown violent extremism, increasing the number of agents and analysts working on it in field offices across the country. The FBI told Horowitz that it had arrested 65 people who were planning to conduct terrorist attacks in the U.S. from January 2015 to December 2018.