The brothers who carried out the Manchester Arena attack "did not act alone" and others who knew or suspected the bomb plot are still "at large", a public inquiry heard.
Patrick Gibbs QC, representing British Transport Police (BTP) at the public inquiry into the atrocity said it was "obvious" suicide bomber Salman Abedi and his brother Hashem, from south Manchester, must have had help.
Mr Gibbs told the inquiry sitting in Manchester: "But what of the other potential murderers, we don't yet know their names."
He cited the "intricate and lengthy and carefully planned preparations" of the brothers, detailed in the inquiry earlier and during the trial of Hashem Abedi.
He added: "It will be obvious, I suggest, to all of us that those brothers did not act alone.
"They must have received technical help and financial help and training and support from other people.
"Other people must have known or at least suspected what they were up to and those other people are at large."
Mr Gibbs suggested to inquiry chairman, Sir John Saunders that the "mountain" of money spent on the public inquiry would be worthwhile if other people were brought to justice.
He added: "If one or more of those accessories to mass murder can by your process be run to ground and brought to justice then there will be no question but that it has been money well spent.
"Put another way, your inquiry, I suggest, is probably our last opportunity, to track down those without whose help and inspiration these murders could not have been committed."
The public inquiry, scheduled to last into next spring has already heard about the Abedi brothers "radicalised family", his father Ramadan's involvement with Islamist groups in his native Libya and their connections and associations with known terrorists in the UK.
After months of planning, Salman Abedi detonated a home-made bomb in his rucksack packed with shrapnel in the City Rooms, the foyer to the Arena as the crowd emerged at the end of an Ariana Grande concert at 10.30pm on May 22 2017.
Twenty-two bystanders were murdered and hundreds injured.
'Carousel of blame'
The inquiry, now in its third week, heard on Tuesday the opening statements from institutions addressing responses to observations and failings made in expert reports, not yet made public, about their response to the bombing.
More than one lawyer for the organisations involved warned against the use of "hindsight" to judge their response to the terror attack.
Mr Gibbs said, though BTP was responsible for policing Manchester Arena as it is on the Victoria railway station site, they held no intelligence about Abedi, though he was known to the security services and counter-terrorism police.
And despite Abedi's three trips to the Arena in the days before the attack on "hostile reconnaissance" a young man with a backpack at a railway station was not unusual, the inquiry heard.
It is only the "agony of hindsight", Mr Gibbs suggested, which now draws attention to him, caught on "macabre" CCTV, when he returned the final time on the night of the bombing.
He said earlier two BTP officers had missed Abedi "by a matter of seconds" at 8.48pm when they conducted a check on the toilets while the bomber was in there with his backpack.
He said "overall" the BTP response was "extremely good" and he had "nothing negative" to say about other organisations, such as Greater Manchester Police, fire or ambulance services.
He said a "carousel of blame" had been "sent spinning" by Jonathan Laidlaw QC, representing Showsec, employed by SMG, the Arena operators, to provide security.
Mr Gibbs added: "I understand commercial organisations have shareholders and insurers and so on."
Earlier Mr Laidlaw had said it was "not a question of buck-passing" but Showsec provided crowd management services with security only one element of its job, typically employing students, part-time, to fill such roles and there were limits to what could be expected of its staff in terms of counter-terrorism.
A member of the public had alerted Showsec staff to Abedi acting suspiciously but this was not passed on to the control room and no action taken before he detonated his device.
The hearing was adjourned until Wednesday morning.