Cesar Altieri Sayoc, a 56-year-old Florida man, was arrested Friday in connection with this week’s mail bombs sent to critics of President Trump. Notably, he’s the only person who has been charged. If that remains true, this will be regarded as a case of lone-wolf terrorism — a type of attack that has increasingly concerned law enforcement agencies since the 1990s.

As of 2015, lone wolves accounted for 6 percent of all terrorists in the U.S. — but they were responsible for 25 percent of all U.S. terrorist attacks. That’s according to a report by the National Consortium for the Study of Terrorism and Responses to Terrorism (START), the organization that manages the Global Terrorism Database. Historically, lone wolves have been able to avoid arrest for longer periods of time than terrorists who act in groups.

Lone-wolf terrorists are also the more deadly terrorists, according to a 2017 study published in the journal Terrorism and Political Violence. That’s largely because we have strong counterterrorism law enforcement operations that make it difficult for group terrorism to operate effectively. Lone wolves tend to be more educated and more socially isolated than other kinds of terrorists.

The use of package bombs by terrorists is generally rare but has waxed and waned over the decades with a peak in the 1990s, according to data released Thursday by START. The majority of package bomb attacks happen in Western Europe. Worldwide, these kinds of attacks tend to work less often — with a 39 percent success rate compared with 89 percent for all kinds of terrorist attacks. Package bomb attacks also tend to be part of a series, with 29 percent involving multiple targets attacked simultaneously or in close succession, compared with 14 percent of all terrorist attacks. Both these trends held true in this case, in which multiple people were targeted and none of the bombs went off.

One other thing that also may be consistent between the research and this week’s events: There are connections among lone-wolf terrorism in the U.S., bombings and right-wing extremism. While authorities are not yet willing to talk about Sayoc’s possible motivations, they believe that he owns a van covered in bumper stickers suggesting strong conservative leanings. According to START, when lone-wolf terrorism began to emerge in the United States in the 1990s, it was strongly associated with a “leaderless resistance” model of “uncoordinated violence” advocated by right-wing extremists. In a 2013 report, the same organization noted that 48 percent of all lone-actor terrorist attacks in the U.S. between 1992 and 2010 targeted abortion providers and advocates, and that bombing had been the method of choice for lone actors 36 percent of the time.