Washington, Mar 5 (AFP) Surveillance of "homegrown" US extremists by the FBI was flawed until 2019, a watchdog report said Wednesday, saying that the weaknesses may have allowed deadly attacks to occur.

The Department of Justice Office of the Inspector General (OIG) report said that the FBI had closed several assessments of suspects who went on to launch attacks.

"Since September 11, 2001, HVE (home-grown violent extremists) have carried out over 20 attacks in the United States, some of which occurred after the FBI closed a counterterrorism assessment or investigation," it said.

"The FBI had not taken a comprehensive approach to resolving deficiencies."

According to the FBI, the extremists were jihad-inspired individuals radicalized in the United States and not receiving orders from abroad.

The report focused on one of the Boston marathon bombers, Tamerlan Tsarnaev, who planted two home-made bombs near the finish line of the race in 2013, killing three people and injuring 264 others.

It said database searches and questioning of Tsarnaev, his parents and Tsarnaev's former girlfriend and wife "would have resulted in a more thorough assessment" before the attack.

The FBI also closed an assessment of Omar Mateen, who later shot and killed 49 people in Orlando's Pulse night club in 2016.

The case agent "was also assigned to work criminal matters and had little formal counterterrorism training," the report said. (AFP)

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