Troubling anniversaries linked to U.S. extremists have fueled speculation about who is behind the Boston bombings even though investigators have yet to publicly cast blame on any culprits.
The twin bombs left at the marathon's finish line Monday, which killed three people and wounded more than 175, contain disturbing details that could be seen as aping Al-Qaeda.
President Barack Obama has clearly accused terrorists of staging the attacks. But he also insisted that neither domestic, nor foreign militants, nor a "lone wolf" individual, had yet been implicated.
Still, the date alone -- April 15 -- leaves plenty of room for speculation.
Monday was the deadline for Americans to pay taxes, a day hated by the far right. In Boston, it was also Patriots' Day -- the anniversary of the first battles against British forces in the American Revolutionary War.
This Friday will mark the 20th anniversary of the end of the siege of a radical sect at Waco, Texas, in which 76 people died. A bombing in Oklahoma City by Timothy McVeigh, a hero to extremists, took place on April 19, 1995. It claimed more than 160 lives.