Aside from threatening nuclear war against the United States, North Korea is also suspected of secretly developing a vast biological-weapons program that could unleash fear and death in crowded cities, a Harvard University study warns.

“North Korea is likely to use biological weapons before or at the beginning of a conflict to disrupt society and create panic, incapacitate societies, and/or cause a significant military diversion,” says the Belfer Center for Science and International Affairs at the Harvard Kennedy School.

The study also warns that North Korea could deliver such weapons in several manners, including janitors.

“It is theoretically possible that North Korean sleeper agents disguised as cleaning and disinfection personnel could disperse BW agents with backpack sprayers,” the Harvard report said.

North Korean dictator Kim Jung Un is believed to be developing biological agents such as anthrax, cholera and smallpox, London’s Daily Mail reported.

The Harvard report also notes that the exact extent of the biological-weapons program is hard to determine because many of the agents also are used in agricultural programs.

The study comes as Kim unleashes threats against the United States almost daily, while Washington prepares to defend against a surprise attack. The U.S. Air Force has put its fleet of B-52 bombers, which can carry nuclear weapons, on 24-hour alert for the first time since the end of the Cold War.