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A new report from the British security service M15 looks at four terrorist attacks that took place in the U.K. within a four period earlier this year and claims that authorities had information about the 22-year-old suicide bomber who killed 22 people at an Ariana Grande concert last May. (You can read the full report here, via Rolling Stone.)

According to the report, Salman Abedi was a subject of interest to the U.K. police but was not under investigation at the time of the Manchester bombing. However, the report also states that 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.”

That intelligence—the report does not specify what it was—could have led the police to open an investigation into Abedi, had it been better understood at the time: “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.”

However, the report stops short of saying that the attack could have been prevented if the police had been investigating Abedi at the time of the shooting:

Salman Abedi was a closed SOI at the time of his attack, and so not under active investigation. MI5 nonetheless came by intelligence in the months before the attack which, had its true significance been properly understood, would have caused an investigation into him to be opened. It is unknowable whether such an investigation would have allowed Abedi’s plans to be pre-empted and thwarted: MI5 assesses that it would not.

“Unknowable” is probably the most important word in that sentence—there’s truly no way of knowing what could have happened had authorities been actively looking into Abedi in May, especially given the fact that M15 has not revealed what exactly the police glanced over. One silver lining: In a joint statement with M15, the Metropolitan Police Commissioner said that “nine terror plots had been thwarted since March,” according to CNN.