An updated terror bulletin was issued by the Department of Homeland Security (DHS) on Thursday, warning Americans about the possibility of weaponized drones, chemical attacks and the continued targeting of commercial aircraft.

“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,” according to the bulletin.

The color-coded threat system has been replaced by the National Terrorism Advisory System bulletin, which is now being used to give the public and local law enforcement a summary about ongoing terror threats and the potential for new ones.

“The current bulletin introduces unmanned aircraft systems as potential threats and highlights sustained concern regarding threats against commercial aviation and air cargo,” said DHS acting press secretary Tyler Houlton in a statement.

There’s been an “uptick in terrorist interest” in using unmanned aerial systems as weapons in the United States and other western countries, a senior DHS official told reporters. These tactics have been used by terrorists on the battlefield, and the department wants to “guard against those tactics being exported to the west,” said the official.

DHS wants to be “forward leaning” about seeing what terrorists are doing overseas and tactics they might adopt in the future, the official noted.

Concerns about terrorists targeting the U.S. aviation sector are mounting. “[T]errorists continue to target commercial aviation and air cargo, including with concealed explosives,” reads the updated bulletin.

The DHS has been implementing wide-ranging security measures for all airports and airlines that fly directly to the United States. In June, the administration announced “enhanced screening” of passengers and their electronic devices, as well as “seen and unseen” security around the aircraft and inside the airport, according to a report by ABC News.

Terrorists still see “aviation as the crown jewel target,” said former DHS Secretary John Kelly, now the president’s chief of staff, at the time of the announcement.

The new measures, which are being rolled out in phases, are aimed at detecting concealed explosives, insider threats and identifying suspicious passengers.

Current acting DHS Secretary Elaine Duke has been urging countries to adopt these measures on all flights, not just those that are direct to the United States, according to a DHS official.

The new bulletin also carries a warning about the use of “poisons or toxins.” According to the DHS official, there has been increased chatter about the use of chemicals throughout terror networks.

The “big picture” is that the homeland security fight is shifting, said the DHS official. The department’s response to the terror threat is adapting, since ISIS is close to defeat in safe havens but continues to have branches and affiliates around the world, according to the DHS.

The DHS is focused on the next phase of the fight, according to the official.