Manchester Arena bomber Salman Abedi was challenged by a member of the public shortly before he detonated the bomb that killed 22 people at an Ariana Grande concert three years ago.
The details of the confrontation emerged as a public inquiry into the atrocity began in Manchester.
The concertgoer, who can be named only as witness A, said he approached a man resembling Salman Abedi because he looked “out of place” on the night of the 22 May 2017 attack.
Witness A said the suspicious man was carrying a large rucksack in the crowded venue.
Paul Greaney QC, counsel to the inquiry, said in the opening hearing: “(The witness) asked the man, what have you got in your rucksack, but got no reply.
“(The witness) then said, ‘It doesn’t look very good you know, you with a bag in a place like this. What are you doing?”
The man is said to have replied: “I’m waiting for somebody, mate. Have you got the time? What time is it?”
Witness A said he was “fobbed off” by security worker Mohammed Agha when he warned him about the suspicious man, the inquiry heard.
The witness had spoken to Mr Agha at 10.14pm, some 17 minutes before the detonation.
Mr Agha then spoke to his colleague Kyle Lawler about the matter eight minutes before the bomb went off.
But neither security control, nor anyone else, was informed about the suspicious activity, the hearing was told.
Mr Lawler is said to have tried to radio his security colleagues but could not get through.
He then spotted the man get up and start walking towards the arena entrance.
Mr Lawler said in a statement: “I just froze and did not get anything out on the radio. I knew at that point it was too late.”
Mr Greaney said the inquiry would examine if mistakes were made: “Why did Mohammed Agha and Kyle Lawler not inform the control room or anyone else between 10.14pm and 10.31pm about the report from witness A of a suspicious male, with backpack, on the mezzanine level of the city room?
“If their failure to do so was culpable, was that the result of inadequate training and/or instruction or, instead, the consequence of individual error of ineptitude?”
Another man with concerns about Abedi reported him to a police officer.
Mr Greaney added: “Whether these represent missed opportunities to prevent what happened that night, to reduce its scale… (are) an issue of very considerable importance.”
The inquiry also heard that fire and ambulance crews were delayed getting into the foyer – a matter that experts believe is of “significant concern”.
Survivor Sean Gardner has told Sky News that he and many others in the foyer felt abandoned for at least 25 minutes after the explosion: “It has obviously left some really deep scars.
“There was always going to be a five or 10 minute gap but it was 25 minutes before we got any assistance.
“We need to hear the reasons for the delays and hopefully they were the right decisions.”
A previous report by Lord Kerslake found that Greater Manchester Fire and Rescue Service played “no meaningful role” for nearly two hours and there was “little awareness of what was happening at the arena”.
The inquiry also heard that Salman Abedi’s older brother Ismail received a text message from family in Libya, three and a half hours before the attack offering him God’s blessings, the inquiry heard.
He has refused to take part in the inquiry in case he incriminates himself.
The inquiry is expected to run until spring 2021.