A Google earth 3d search of the buildings at ARG International do not appear to show any fixed exterior ladders for roof access. There also do not appear to be any roof access hatches on any of the facility roofs. How did he get on the roof...~ fifteen feet high?
I’m using my phone, so by resolution may be poor.
Brought a ladder?
Had one prestaged?
Knotted Rope - IF he was physically fit to climb such. Or a rope plus rope ladder.
In his younger days, I saw my Dad (Forestry Prof.) use the latter to get up into trees.