I'm glad I'm not the Secret Service. A drone swarm has to be rapidly becoming one of their scary scenarios for something like inaugurations.
One can probably be picked up from an army surplus warehouse somewhere in the former Soviet Union/(some current conflict zone), for an ~$X,000 USD bribe, or alternatively, manufactured by a literate, mildly motivated individual with a high tolerance for personal risk, a welder, a lathe, a tool-shed, and an outhouse in the mountains.
Given that we live in a world where gunpowder has been invented, you will have to pardon me if I'm more concerned about the dangers of lunatics with mundane explosives, and delivery systems thereof.
Or, even use our own mission planning software against us - ATAK[1][2].
[1] https://en.wikipedia.org/wiki/Android_Team_Awareness_Kit
[2] https://play.google.com/store/apps/details?id=com.atakmap.ap...
Mortar is not a precision weapon, it's a Monte Carlo prop.
And with a kill zone of 20-30 meters... you can miss by quite a lot and still do a ton of damage.
Mortar attacks for this are not unprecedented†, but are not terribly effective.
[†] https://en.wikipedia.org/wiki/Downing_Street_mortar_attack#T...
But, for inflicting terror and great loss of life in a populated area - they could be used very effectively.
https://en.wikipedia.org/w/index.php?search=IRA+%22mortar+at...
Amongst other things, shooting down a helicopter, and having a go, separately, at the queen and the prime minister.