- An element of surprise;
- Assailants who conceal themselves, their weapons or their intents;
- The suddenness of the attack;
- A lack of provocation.
Attackers may attempt to lure lone officers into a situation with the intent of killing or injuring them in a pre-meditated attack. But more often, the assaults are spontaneous -- the result of an attacker taking advantage of an unexpected encounter with an officer.
The killing of Marconi appears to fall into this latter category. He was shot in the head while he sat in his patrol car while writing up a traffic ticket for a motorist evidently unconnected to the attacker.
Like many categories of crime, ambushes on officers have become rarer since the early 1990s. In 1991, for instance, there were more than 500 ambush attacks (fatal and non-fatal) on officers. By 2015, that number had fallen to 240 assaults, according to official FBI statistics.
Overall, killings of police officers are down too, according to the FBI. In 2011, for instance, 72 officers were feloniously killed in the line of duty. By 2015, that number had fallen to 41.
This year is shaping up to be a more violent year for officers than 2015. Overall, officer fatalities from all causes including accidents are up 16 percent from last year, according to the National Law Enforcement Officers Memorial Fund. But the number of officers fatally shot by suspects is up by 67 percent year over year.
These figures are subject to significant year-over-year fluctuation in part because these incidents are so rare. When only 10 or 20 incidents happen in a given year, as with ambush killings, it's easy for one or two outliers to drastically affect the numbers.
All of these factors, and their myriad causes, are a much bigger driver of officer safety than the policies of whoever's sitting in the White House. That's been true during the Obama administration, and it will be true during the Trump administration, as well.