Salman Abedi had travelled out to Istanbul, a known gateway for extremists heading to Syria, in May 2016, the year before the attack, the inquiry into the bombings has heard.
Information had twice come to MI5's attention that he wanted to travel to Syria but was "discounted" when he did not go, and they were also told that he held "pro-ISIS views."
MI5 admitted that information had come to their attention in "mid 2016" that led to them consider re-opening the case, but a meeting was not scheduled to consider doing so until 31 May 2017, nine days after the attack.
The information about Abedi's links to Syria emerged as MI5 gave its most detailed account of what they knew about the bomber, three years after the attack.
The Security Service also had two further pieces of information which is so critical to the public inquiry it will have to be heard behind closed doors.
MI5 said that it "understands that there will be people who doubt that it has disclosed everything" to the inquiry into the bombing which killed 22 people in May 2017.
Cathryn McGahey QC, for the Security Service, said "the very nature of MI5's work is secretive and the fact that we cannot make everything public inevitably increases suspicion."
But she pledged: "There is no question of secrecy bring used to conceal failure."
Meanwhile, the inquiry was also told the fire service response to the Manchester Arena bombing was "neither adequate nor effective".
Its lawyer Andrew Warnock QC said it was "unacceptable" that Greater Manchester Fire and Rescue Service (GMFRS) took more than two hours to attend the incident and apologised to the families and victims.
He said the fire service had trained for the wrong type of terrorist attack and no one believed they were in control on the night of 22 May 2017.
The inquiry was told they had a "misinformed and skewed understanding of what was happening" and a lack of leadership while a senior officer drove from his home into central Manchester.
Mr Warnock said: "No one from GMFRS believed themselves to have command of the incident and no one took charge of the response in its initial stages. That is clearly a serious failing."
The fire service received "virtually no contact" from Greater Manchester Police at all prior to its arrival at the arena and there were failures in communication with the ambulance service, who "were also obviously aware that GMFRS were not at the scene", Mr Warnock told the inquiry.
Andy Berry, the fire service liaison officer, had to travel "some considerable distance" from his home to central Manchester - "a problem compounded by roadworks and closures which he also had to navigate whilst dealing with this incident" and that "did not make his job any easier".
Silence from other emergency services "fed an assumption" that the police "must be busy dealing with an ongoing terrorist situation, which would explain their lack of communication".
By the time the fire service arrived, the last casualty had been evacuated from the City Room, the site of the explosion, an hour earlier.
Mr Warnock told the inquiry: "In relation to the fire service response on the night, we say at the outset that GMFRS accepts and agrees with the conclusions of the fire and rescue expert that its initial actions in response to the arena bombing were neither adequate nor effective.
He added: "The families and injured deserve answers to the questions which they have and we will do all we can to provide them."