The lesson of Manchester is you don't need to get on a plane, into a secure area, or past a search.
Think about that the next time you're standing in the cattle chutes with your shoes in your hand hoping you don't get selected for extra screening. All those people in line with you haven't been screened yet either. That rolling suitcase the next guy has could be his underwear and socks for a week or it could be another nail bomb like the one at Manchester.
If it's organized, terrorist #1 puts his suitcase on the conveyor and detonates it as it goes into the machine. Everyone in the line that's still intact turns and runs back up the concourse. Terrorist #2 and #3 are positioned at the top of the concourse and wait until the panicked crowd approaches, then set off their suitcases for maximum effect.
If it's really well organized, there's a big truck bomb in one of the ambulances that responds in the chaos of the next half hour.
"A strict observance of the written law is doubtless one of the high duties of a good citizen, but it is not the highest. The laws of necessity, of self-preservation, of saving our country when in danger, are of higher obligation. To lose our country by a scrupulous adherence to the written law, would be to lose the law itself, with life, liberty, property and all those who are enjoying them with us; thus absurdly sacrificing the ends to the means." --Thomas Jefferson