The FBI missed opportunities to stop domestic terrorists from killing Americans because they failed to conduct follow-up investigations of people identified as violent extremists, the Justice Department watchdog said in a scathing report released Wednesday.

At least six terrorists who carried out separate attacks killing 70 people had been on the FBI’s radar, but agents quickly closed the cases after concluding they were not national security threats, Justice Department Inspector General Michael E. Horowitz said in the report.

The lapses in the bureau’s assessments allowed perpetrators of some of the most deadly attacks in recent history to fall through the cracks. Among them were:

⦁ Omar Mateen, who killed 49 people at the Pulse nightclub in Orlando, Florida, in 2016.

⦁ Tamerlan Tsarnaev, who killed three people at the Boston Marathon in 2013.

⦁ Nidal Hasan, who massacred 13 people at Fort Hood, Texas, in 2009.

⦁ Esteban Santiago, who killed five people in a 2017 attack at the Fort Lauderdale-Hollywood International Airport.

“The FBI has acknowledged that various weaknesses related to its assessment process may have impacted its ability to fully investigate certain counterterrorism assessment subjects, who later committed terrorist acts in the United States,” Mr. Horowitz wrote.

The inspector general’s report is the latest black eye for the bureau, which has been besieged by allegations of political taint and questions of competence.

“Anytime there is criticism, of course, it undermines the public faith in the bureau, and that can never be good because the FBI depends on the public trust,” said Lewis Schiliro, a former head of the FBI’s New York field office.

Still, Mr. Schiliro said, the blame goes to the FBI bureaucracy, not the agents, who are hamstrung with limited resources and 90-day time constraints on preliminary investigations if no immediate threat is found.

“It’s going to be a judgment call,” he said about closing cases early. “Agents get hundreds of cases a day, and you are limited where you can apply your resources.”

Even after the FBI discovered lapses in its assessments of potential terrorist threats, field office managers failed to properly implement changes or conduct consistent oversight of counterterrorism investigations, the report said.

Roughly 40% of the FBI’s counterterrorism assessments went unaddressed for 18 months, even after bureau officials discovered investigative lapses, Mr. Horowitz wrote.

An FBI spokeswoman did not respond to requests for comment.

The FBI first investigated Mateen in 2013, three years before he carried out the deadly Pulse shooting. Agents closed the case months later and did not properly address Mateen’s history of mental illness, the report said.

Agents investigated Tsarnaev ahead of the Boston Marathon bombing. Even after an internal bureau database flagged Tsarnaev, agents closed the probe after concluding he had “no nexus to terrorism.”

The inspector general said the bureau bungled the case of Elton Simpson, who tried to ambush a Garland, Texas, art exhibit featuring cartoon images of the Prophet Muhammad, the central figure of Islam.

Although agents received information related to Simpson, they determined he was not a significant threat.

Mr. Horowitz said that even after the FBI sought to address the problem, it failed to conduct the necessary oversight to implement recommended changes.

Roughly one year after procedures were implemented, two field offices were not using them.

“As a result, potential terrorist threats were not mitigated for more than one year,” Mr. Horowitz wrote.

The inspector general offered seven recommendations for the FBI to develop a “comprehensive strategy” for investigating terrorist suspects and keeping agents up to date on those techniques.

Recommendations include implementing guidelines from a 2017 internal bureau review of its Guardian threat assessment system, providing clear guidance to field offices on investigative steps, and establishing a plan to assess threats with mental health concerns.

In a response included in the report, an FBI official said the bureau agrees with the recommendations.

“We agree it is important to continue to improve the assessment process, provide adequate guidance, training and program management for all Guardians and those specifically addressing homegrown violent extremists,” wrote Suzanne Turner, an FBI section chief in the inspection division.

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