MI5 didn't tell police about intelligence 'highly relevant to the planned attack' just months before the Manchester Arena bombing, the public inquiry was told.
At the time, suicide bomber Salman Abedi was a 'closed subject of interest' but 'continued to be referenced from time-to-time' in intelligence reports, an official report into the terror attacks in the UK in 2017 stated.
The inquiry heard on Wednesday that on two separate occasions in the months before the attack, intelligence on Abedi was received by MI5 which was not 'fully appreciated' at the time.
It was assessed to 'relate not to terrorism, but to possible non-nefarious activity or to criminality on the part of Salman Abedi'.
But the inquiry was told the two items of intelligence were later, in retrospect, said to have been 'highly relevant to the planned attack'.
Detective Chief Superintendent Dominic Scally, head of Counter Terrorism Policing North West (CTPNW), said in evidence they were not told of the intelligence by MI5 prior to the attack.
"The first of those we did not receive before May 22, 2017," he said.
Nicholas de la Poer QC, counsel to the inquiry, asked him: "So far as Counter Terrorism Policing North West is concerned, did MI5 tell it about the second piece of intelligence prior to the attack?"
Mr Scally replied: "No."
'Closed' inquiry hearings to discuss the issue of 'preventability' on the part of Salman Abedi start next week - three weeks of hearings not open to the press or the victims' families.
Mr de la Poer said he would be exploring with Mr Scally - in the closed sessions - his view on 'whether that should have been communicated and the degree, if any, that you assess it was capable of making a difference' in preventing the attack.
Twenty-two people were killed and hundreds more injured when Abedi detonated a bomb after an Ariana Grande concert at the venue on May 22, 2017.
Sir John Saunders, the inquiry's chairman, said he would consider whether further details could eventually be revealed publicly.
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The 'closed' sessions are being held for issues of national security.
Sir John said: "If I decide it can be broken out [from the closed hearings] I can assure you that it will be."
The exact nature of the intelligence hasn't been revealed.
Post-2017 reports 'raised concerns as to how well MI5 and counter-terror policing worked together' and 'problems around the sharing of MI5 information with counter-terror policing', the inquiry was told.
But Mr Scally said it wasn't his 'experience' that there was an 'imbalance in the relationship' between them.
Previously, referencing a period between 2010 and 2017, Mr Scally described the relationship as 'excellent'.
"There will be rarely a day that goes by where I do not have contact with our partners," he said. "That relationship has improved. We share more."
Mr Scally, before giving his evidence, said it was 'a source of great regret' that counter-terror policing failed to prevent the attack.
He expressed his sympathies to all those affected by the bombing and said: "We work hard everyday to protect our communities from terrorism. I know it is a source of great regret that we failed to prevent this attack."
Abedi was classed as a 'subject of interest' - SOI - by MI5 for six months from January 2014 to July 2014, after which a residual risk of 'low' was ascribed to him, Mr Scally said.
But after that, he was linked to other SOIs and in November 2014, a mobile phone was seized which was later attributed to Abdalraouf Abdallah - a now convicted terrorist recruiter who is due to give evidence to the inquiry at a later date and who Abedi visited in prison.
Abedi, the inquiry heard, sent a 'Jihadi nasheed' text message to Abdallah, on November 12, 2014, speaking about 'soldiers of sacrifice advancing forward and crushing the stronghold of their enemy', the inquiry was told.
On the same day, two more text messages were received by the phone, one containing the words 'Salman Abedi' and the other, the word 'AbEdi' with a capital E, the inquiry heard.
They weren't 'acknowledged' by Abdallah, who was jailed in July 2016 for terror offences facilitating the movement of people to Syria.
Abedi was also 'indicated' to have been in contact with one SOI - said to have been a longstanding subject of interest because of his affiliation with an extremist group in Libya - in mid-2015.
But Mr Scally said he didn't believe counter-terror policing was told of that intelligence by MI5 either.
And in September 2015, the inquiry heard Ismail Abedi - the elder brother of Salman and Hashem Abedi who is now not in the country and has said he won't give evidence to the inquiry - was 'port stopped'.
On his devices, extremist terrorist material was found, with one item described by experts as the 'seminal ISIS text', the inquiry heard.
Lawyers for the families have raised a number of issues questioning MI5's handling of Abedi.
They include why Abedi was not subject to a 'port stop' when he returned to the UK from Libya to execute the bomb plan, why he wasn't further investigated post-2014 because his multiple contacts with suspected terrorists and his visits to Abdallah in prison.
It's been suggested the 'cumulative' effect of multiple pieces of intelligence about the bomber, and the fact MI5 and police recognised young men in south Manchester were at risk of radicalisation, should have led to him being made an SOI again after the case on him was closed in 2014.
Mr Scally said based on the intelligence available at the time, he didn't believe Abedi should have been referred to the Government's anti-extremism, de-radicalisation programme Prevent - an issue Sir John said would be further examined.
The witness told the hearing that contact with SOIs alone did not merit referral to Prevent or an investigation by the authorities.
He said 'tens of thousands' hold extremist views, and referenced a threshold for police and security services taking action.
The inquiry continues.
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