A bulletin from the U.S. Department of Homeland Security published Thursday suggests that terrorists are turning to new tactics as technology advances.

The department said fighting terrorism these days has been the most difficult since the Sept. 11 terror attacks. The reason is more ease and access to new technologies. 

The latest warning from the department suggests that terrorist may start using drones to carry out attacks. The agency said in its advisory bulletin that overseas terror groups are using battlefield experience to pursue new tactics that included unmanned aircraft and chemical agents.

The DHS said homegrown terrorist are communicating with each other on encrypted social media applications. Terror groups are also urging recruits to use simple weapons, such as knives, vehicle and homemade explosives, to launch attacks, the agency said.

The full DHS bulletin can be found below: