Six explosive devices were sent to leaders of the Democratic Party, including two former Presidents, and to CNN and George Soros, who are both the targets of criticism from President Donald Trump and others on the right. The motive is unknown.
But the even bigger question, of course, is: Who is the person or persons behind these attacks?
Determining that may take some time if the bombmaker or -makers is skilled in covering their tracks.
Recall that the "Unabomber" Ted Kaczynski dispatched a series of bombs in the mail over the course of 17 years to academics and businessmen who he felt were opposed to his obscure neo-Luddite beliefs.
Kaczynski killed three people and injured 23 others before the FBI finally arrested him in 1996. That arrest was instigated by a tip from Kaczynski's brother.
Recall also the anthrax-laced letters that were sent to media organizations and congressional offices in the weeks after 9/11. Five people died from anthrax inhalation. It took the FBI seven years to finger leading American microbiologist Bruce Ivins for the attacks. Ivins committed suicide as authorities closed in.
Kaczynski and Ivins were both operating alone and were not part of a group.
These "lone actor" cases are often harder for law enforcement to solve than when there is a larger conspiracy involving multiple terrorists.
That said, today there are more tools available to law enforcement than in the earlier cases, which can help them find terrorists -- not least, almost ubiquitous surveillance cameras and more precise DNA technology.
Because of the 9/11 attacks, Americans often frame "terrorism" around jihadist terrorist operations. But the fact is there are other forms of political violence in the United States, which is why it was refreshing to hear top New York officials on Wednesday describe the six explosive devices that were sent to the leading Democrats, George Soros and CNN as terrorist acts.
Recent terrorism has emanated from the left and right. In June 2017, 66-year-old James T. Hodgkinson III attacked Republican members of Congress who were practicing baseball in Alexandria, Virginia, injuring five, including House Majority Whip Steve Scalise, who was gravely wounded.
Hodgkinson had posted on Facebook: "Trump is a Traitor. Trump Has Destroyed Our Democracy. It's Time to Destroy Trump & Co." Hodgkinson was shot by police officers and died after his attack.
Seven months earlier, on December 5, 2016, believing a debunked right-wing conspiracy theory that Hillary Clinton was running a child sex ring out of the basement of the Comet Ping Pong pizza joint in northwest Washington, DC, 28-year-old Edgar Welch of Salisbury, North Carolina, went to Washington to "self-investigate."
Welch walked into the popular pizza restaurant carrying an assault rifle and started firing shots. He pointed the firearm in the direction of a restaurant employee, who fled and notified police who arrested Welch. Welch told investigators that he had come armed to help rescue the children.
Since 9/11, while 104 Americans in the United States have been killed by jihadist terrorists, 73 have been killed by far-right terrorists and eight have been killed by terrorists motivated by black nationalist ideology, according to New America, a research institution.
The bombs that were sent to the Democratic Party leaders, George Soros and CNN remind us that political violence in the United Sates can come from a wide range of ideologies and motivations.