In recent times concern has grown that groups or lone militants could carry out terrorist attacks using unmanned aerial vehicles (UAVs), more commonly known as drones.
Two recent events have raised further alarms. On 4 August, two drones apparently carrying explosives detonated in Caracas, without killing anyone, while Venezuelan President Nicolás Maduro was giving a speech to the National Guard. On 26 September, two people were arrested in Copenhagen and charged with being members of a broader network that shipped drones and other supplies from Denmark to the so-called Islamic State (IS) for use in combat.
Drones have clear strengths in fighting. They fly without a human pilot aboard and, unlike rockets and missiles, they can perform controlled landings and undertake repeated flights.
Initially the prerogative of states and their armed forces, including in operations against terrorists, this technology has spread to companies and finally individuals.  The barriers to entry have become generally low.
It is therefore not surprising that drones are also used by terrorists and insurgents in a process of learning and innovation. They can appeal to militant non-state actors for the same reasons they are attractive for states: they offer a relatively inexpensive way to achieve relevant objectives, including attacking a target, without risking personnel. To some extent, as has recently been emphasized, “drones have democratized warfare. For the first time in history, nonstate actors have an air force”. 
In general, even large unmanned aircraft do not pose a strategic threat to the armed forces of a powerful state in a war. In addition, large combat UAVs are expensive. The US “MQ-9A Reaper”, for example, costs more than $16 million. Sums this large are beyond the reach of most terrorist organizations. Even armed groups with ample financial resources, such as IS before the collapse of its “caliphate”, have generally preferred cheaper weapons, specializing in the innovative and improvised use of technology that is not particularly advanced but is easily accessible. 
At the tactical level, though, drones are a threat, and cheap too. Smaller commercial drones are affordable and can fly at low altitudes to escape detection by radar systems, posing a significant threat to ground forces and civilians.
An important study on the use of drones by terrorist organizations published in October 2016  identified four armed groups with fully fledged and dedicated “programs”: Hezbollah, Hamas, Jabhat Fatah al-Sham (which merged with other groups to become Hayat Tahrir al-Sham in January 2017), and IS. It is worth noting that all four of these groups operate in the Middle East and can be described as “Islamist”, although they certainly have different ideological orientations and objectives.
Hezbollah, benefiting from Iran’s direct assistance, and Hamas have been particularly notable pioneers in exploiting the opportunities offered by drone technology. IS developed its program a few years later, in 2014-2015, but with impressive rapidity. In Syria and Iraq, Abu Bakr al-Baghdadi’s organization mainly used commercially available drones modified for military purposes, but also experimented with in-house construction and assembly. In particular, IS creatively combined sophisticated commercial-off-the-shelf (COTS) technology with low-tech components. 
Initially, the IS drones were used for surveillance and propaganda. But violence was also an objective. It was reported in the autumn of 2016 that the organization had managed to arm drones by attaching explosives that could be released when a drone approached a target. In October 2016 two Kurdish Peshmerga fighters were killed and two French special forces soldiers were seriously injured  in what was described as the first confirmed case of a lethal drone incident associated with a terrorist organization.
This attack was the outcome of a systematic effort by Baghdadi’s organization, without the help of state sponsors. More details about the program dedicated to drones came to light with the discovery of relevant documents in Mosul, Iraq, in 2016. These materials, produced mainly in 2015, show that the armed group was committed to developing and institutionalizing a centralized program of drone weaponization. 
Drones can also be used by armed groups outside of active theaters of war. In the US, the authorities have already foiled a terrorist plot on the national territory. Rezwan Ferdaus, a young jihadist sympathizer who was born and grew up in the country, was arrested by the FBI in September 2011 for planning an attack on the Pentagon and the Capitol Building with drones.
Despite their utility to terrorists, a drone can be purchased legally or even built from scratch. Drones can also be stolen or acquired on the black market. Some models can also be hacked. 
Once it is in terrorist hands, a drone can perform a function similar to that of a vehicle deliberately launched against a target or, if equipped with an explosive, a car-bomb, with the added advantages of being able to overcome any barriers and obstacles on the ground, all without requiring the sacrifice of militants. In the air, a drone could deliberately enter restricted airspace and try to crush a plane or a helicopter. 
Swarm attacks are another option. Common small multicopters are usually not able to transport enough explosive material to destroy a building. But just one can carry the equivalent of a grenade, so a swarm could inflict serious damage.
Using drones as a delivery mechanism for chemical weapons is an especially sinister option.
Intelligence is another risk. A group or even a lone militant with terrorist intent can use a drone to gather relevant information. The surveillance and reconnaissance activity of a drone with a camera or a sensor can be both more effective and less risky than operations carried out in person.
Moreover, with rapid commercialization and diffusion, such as the development of delivery drones, identifying a drone as suspicious could become difficult in increasingly congested airspace,  especially in urban areas.
With this backdrop, the present may be no guide to the future. So far, outside of conflict areas, drones have not been very dangerous in material terms, even if they have symbolic value and psychological impact. But as the alleged assassination attempt against Maduro suggests, the situation might be very different in the future.
 See, in particular, Nicholas Grossman, Drones and Terrorism: Asymmetric Warfare and the Threat to Global Security, London: I.B. Tauris, 2018
 Nicholas Grossman, “Are drones the new terrorist weapon? Someone tried to kill Venezuela’s president with one”, Monkey Cage, The Washington Post, 10 August 2018.
 Truls Hallberg Tønnessen, “Islamic State and Technology – A Literature Review”, Perspectives on Terrorism, Vol. 11, Issue 6, December 2017: 101-111
 Don Rassler, Remotely Piloted Innovation: Terrorism, Drones and Supportive Technology, Report, Combating Terrorism Center (CTC), October 2016
 Don Rassler, The Islamic State and Drones: Supply, Scale, and Future Threats, Report, Combating Terrorism Center (CTC), July 2018
 Michael S. Schmidt and Eric Schmitt, “Pentagon Confronts a New Threat From ISIS: Exploding Drones,” New York Times, 11 October 2016.
 Don Rassler, Muhammad al-`Ubaydi and Vera Mironova, The Islamic State’s Drone Documents: Management, Acquisitions, and DIY Tradecraft, Perspective, Combating Terrorism Center (CTC), 31 January 2017.
 See, for example, Kyle Wesson and Todd Humphreys, Hacking Drones, in “Scientific American”, 309, November 2013, pp. 54-59.
 For example, Bernard Hudson, “Drone attacks are essentially terrorism by joystick”, The Washington Post, 5 August 2018.
 Among others, Colin P. Clarke, Approaching a “New Normal”: What the Drone Attack in Venezuela Portends, Foreign Policy Research Institute, 13 August 2018.