human
The guns were not automatic, they were semi-automatic that had "bumper stocks" added to them which increased the speed of firing (as I understand it). Now days, people can use a 3-D printer to make parts for just about anything. For that matter, people have made whole fire arms with them. Had his shots hit the explosives he planted by the jet fuel tanks, then the death toll would have been 100 times or more of what it was. (reported)
When someone is as motivated as this guy was to kill a mass of people, they will find a way. Bombs killed here in Alaska, in 1991 Chugiak, Alaskan resident David Kerr and his wife, Michelle, were the victims of a mail bomb. 
Remember the "Unabomber" ? 
"The nail-studded bombs killed Judge Vance at his home in Mountain Brook, Ala., and Mr. Robinson in law office in Savannah in December 1989".(another bomber)
"April 1919, at least 36 booby trap dynamite-filled bombs were mailed to a cross-section of prominent politicians and appointees by Galleanists.
gpt
{"moderation": ["safe"]}