No-one knows yet who carried out the Boston bombings. But one thing is clear: until someone claims they detonated them, home-grown American terrorists cannot be ruled out. And the FBI certainly isn't ruling them out.
April is a highly significant month in the calendar of right wing, anti-government extremists in America.
It's the month when they remember a string of highly significant deadly incidents, beginning with the FBI siege of the compound of a cult in Waco, Texas in April 1993, which resulted in nearly 100 deaths, of federal officials and religious adherents.
Federal authorities mishandled the siege from start to finish, but blame was laid squarely at the door of the cult. Far-right wing Americans never forgot and never forgave.
As a direct result of that, and in order to wreak revenge, Timothy McVeigh and his accomplices bombed a Federal building in Oklahoma City in April 1995, killing nearly 200.
That explosion was initially blamed on Islamist extremists. Some of his co-conspirators have never been caught. McVeigh was executed.
Waco reached its deadly conclusion on Patriots Day and yesterday America celebrated that day. In some respects there is little to celebrate and much to mourn.
The massacre of students at Virginia Tech, when dozens were shot dead by a lone student, happened on Patriots Day, in April. The Columbine School massacre, carried out by two students who mowed down teenagers, was also in April.
Patriots Day is also the day by which all Americans must have filed their income tax. That may not seem much, but in the lexicon of grievances nursed by the far right, it's a grim, almost anti-American moment.
The 26th mile of the Boston Marathon was dedicated to the victims and survivors of the Newtown massacre, two hours away in Connecticut.
It was a massacre that sparked a fierce debate about gun laws; the President leading calls for more restrictions to be placed on gun owners. For America's gun-loving right, that is a red rag to a bull.
The anti-government right trumpets its love of freedom - especially the freedom to bear arms. It dislikes a President it considers its enemy.
If the Boston massacre is the work of these extremists it's a huge step up for them; deliberately targeting ordinary Americans to inflict mass casualties; planting several bombs in a coordinated set of blasts that are modelled on Al Qaeda's tactics.
Of course it may turn out that Islamist extremists are responsible.
But the blasts will lead America to the same soul searching, just in different directions. And President Obama, no matter who did this, will come under fire for failing to protect the homeland.
And Boston, where the plotters of 9/11 met and from where some of the planes took off, will forever remember the day their marathon, which celebrates the very best in human nature, turned into a massacre.