London (CNN) The man who killed 22 fans at a concert by singer Ariana Grande in Manchester in May had been a "subject of interest" to British police and opportunities to stop him were missed, according to an official security review published Tuesday.

Security service MI5 came by unspecified intelligence about Salman Abedi in the months before the Manchester attack, the report by senior barrister David Anderson said.

The information would have caused an investigation into him to be opened had its true significance been properly understood.

Instead, he was not under active investigation when he detonated a suicide device at the Manchester Arena on May 22.

"On two separate occasions in the months prior to the attack, intelligence was received by MI5 whose significance was not fully appreciated at the time," the report said.

"It was assessed at the time to relate not to terrorism but to possible non-nefarious activity or to criminality on the part of Salman Abedi. In retrospect, the intelligence can be seen to have been highly relevant to the planned attack."

The report did not say what that intelligence was.

Reviews of the four UK terrorist attacks between March and June this year were overseen by Anderson to provide assurance to the government that the internal reviews by the police and MI5 were thorough enough.

The report noted that while counterterrorism police and MI5 had gotten most things right, "it is conceivable that the Manchester attack in particular might have been averted had the cards fallen differently."

Khuram Butt, who led the knife rampage on London Bridge last June, in which eight people died, was the only attacker to have been under police surveillance when he carried out his atrocity, the report said.

Butt had appeared in a British television documentary "The Jihadi Next Door" openly supporting ISIS.

Nine terror plots thwarted since March

"Policing and our colleagues in the fight against terrorism will continue to learn and improve," Metropolitan Police Commissioner Cressida Dick said in a statement.

"We need to make rapid progress in implementing the recommendations, many of which require new technology, better infrastructures and resources at a time when the threat from terrorism poses significant challenges for police and security services."

The statement, issued jointly with MI5, said nine terror plots had been thwarted since March.

"There are currently well over 500 counter-terrorism investigations, involving more than 3,000 subjects of interest -- along with a growing pool of more than 20,000 individuals who have previously been the subject of terrorism investigations," the statement said.

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"These investigations cover the full range of terrorist activity, from attack planning to activity that supports or facilitates terrorism -- but a significant proportion involve potential attack planning threats. The tempo is more intense than ever."

UK Prime Minister Theresa May confronted US President Donald Trump about the intelligence leaks, which Trump described as "deeply troubling."

The breakdown of trust between the two countries led to the brief suspension in sharing intelligence.