The weekend bombings in New York and New Jersey and a stabbing spree in the Midwest are the latest example of the new terror threat faced by the United States along with much of the West: diffuse, unsophisticated and very hard to counter.
Just days ago the United States marked the 15th anniversary of the September 11 attacks - the suicide airliner assaults that took months or years of meticulous planning by a well structured organisation acting from outside the US.
But the face of terror in America has changed.
Now, attacks are waged by American citizens who are isolated or acting in small groups and may have no links with Islamist extremist organisations other than visits to websites or contact on social media.
Ahmad Khan Rahami, an Afghan-American, was yesterday arrested following a bombing on Sunday in New York's Chelsea neighbourhood and the discovery of other devices in New York and New Jersey.
In Minnesota, a 22-year-old Somali American went on a stabbing spree at a mall on Sunday, wounding nine.
This kind of attacker is very hard to detect, said Lorenzo Vidino, director of the Programme on Extremism at George Washington University's Centre for Cyber and Homeland Security.
And if you detect them, he asked, "What do you do?
"You can't monitor anybody 24/7 just because he's active on social media" used by extremist groups to get out their message.
And yet the internet often plays the role of "the devil on the shoulder" that repeats "kill, kill" to potential attackers, in the words of FBI director James Comey.
The methods used in attacks these days are often crude - a stabbing in Minnesota and pressure cookers filled with shrapnel and made with flip phones, Christmas lights and explosive compound, in the case of Manhattan, according to the New York Times.
Still, they can kill. And they are enough to trigger panic, scare people and make them suspicious of Muslims in general.
"It is quite possible the bombings could have nothing to do with groups such as the Islamic State or al-Qaeda," said the Soufan Group, a security consultancy founded by a former FBI official, Ali Soufan.
"There is no shortage of domestic extremist groups and actors," it added.
"We have seen in NY and NJ area a few clusters of people, a few groups of friends who radicalised together," said Vidino.
However, a news agency with ties to the Islamic State group said the Minnesota mall rampage was carried out by one of its "soldiers".
In the face of such a diffuse threat, US counterterrorism officials want something very specific from government officials and the internet community: halt the spread of technology that encrypts email and other communications and makes them impervious to eavesdropping, as such intercepts are crucial to fighting extremism.
These technologies give an edge to jihadist groups, Nick Rasmussen, director of the National Counterterrorism Centre, said earlier this month.
"We are going dark," said Comey of the FBI.