The Department of Homeland Security reissued for the seventh time a National Terrorism Advisory System Bulletin, which makes note of terrorists’ potential to use drones and chemical attacks outside of the groups’ occupied territory and conflict zones.

The NTAS Bulletin replaced in December 2015 the post-9/11 color-coded alert system. The higher levels of advisory in the NTAS system are Elevated Alert or Imminent Alert; neither have been used since the system went into effect.

“We continue to face one of the most challenging threat environments since 9/11, as foreign terrorist organizations exploit the Internet to inspire, enable, or direct individuals already here in the homeland to commit terrorist acts. Homegrown terror suspects increasingly rely on technology, such as end-to-end encrypted social media applications, to avoid detection,” the NTAS Bulletin warns, noting terrorist groups “are urging recruits to adopt easy-to-use tools to target public places and events.”

“Specific attack tactics have included the use of vehicle ramming, to include the use of rental vehicles, small arms, straightedged blades or knives, homemade explosives, and poisons or toxins,” the Bulletin continues. “Some terrorist groups overseas are using battlefield experiences to pursue new technologies and tactics, such as unmanned aerial systems and chemical agents that could be used outside the conflict zones. Many of these technologies are readily available. Additionally, terrorists continue to target commercial aviation and air cargo, including with concealed explosives.”

ISIS units in Syria and Iraq were found to be using off-the-shelf drones to conduct attacks and film attacks, and the terror group was also conducting research and development based on downed coalition drones.

DHS noted that “violent extremist media encourages individuals worldwide to launch attacks using all means possible” and that loss of ISIS territory abroad “may encourage homegrown terrorists to carry out acts of violence in the homeland instead of attempting to travel overseas to fight or in retaliation for apparent losses.”

“Additionally, foreign terrorist fighters who have acquired training and battle-tested terrorism experience may flee from terrorist-controlled territories with a desire to conduct attacks elsewhere, including the United States.”

DHS said it “broadly… remains committed to preventing violence and threats meant to intimidate or coerce specific populations on the basis of their religion, ethnicity or identity.”

The Bulletin urges Americans to continue to “travel, attend public events, and freely associate with others but remain vigilant and aware of surroundings.”