..."It is the easiest thing in the world to hook a small piece of explosive to a drone, and send it over an event," Kent Moyer, president of World Protection Group, a California-based private security firm, told Business Insider.

When it comes to protecting dignitaries, or stadium events such as sports and concerts, "nobody is really doing countermeasures against drones," said Moyer.

In fact, the Secret Service has established drone-free zones that prohibit flights within a 30-mile radius of presidential speaking engagements. The Secret Service also jams drone signals near the White House.

Nonetheless, the 20-year-old gunman who wounded Trump at a campaign rally in Butler, Pennsylvania on July 13 used a drone to scan the site just hours before he climbed the roof of a nearby building with a rifle. The FAA did impose restrictions on aerial drone flights for about two hours around the time of Trump's speech, though it is unclear whether jamming was in effect. But given a litany of security foul-ups that day, such as failure to secure nearby rooftops, and lack of coordination between the Secret Service and local police, there is ample reason to doubt whether the rally was drone-proof.

Assassination by drone is hardly new, at least for militaries and spy agencies. The US, Britain, Israel and other nations have done it for more than two decades, using larger drones such as the $30 million MQ-Reaper, armed with laser-guided missiles filled with explosives or even sharp metal blades that shred the target. But the danger now is that an insurgent or extremist can weaponize a $300 drone and strike a person within a half dozen miles with little warning.

Despite the proliferation of small drones in private hands, including terrorist groups such as Islamic State, they have not really been employed for non-state-sanctioned attacks on public figures. But the omens are there, notably in 2018, when Venezuelan president Nicolás Maduro survived an attack by explosive drones while giving a speech in Caracas....