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Troubling anniversaries linked to US extremists have cast a shadow over the Boston bombings even though investigators have yet to blame any group publicly.
The twin bombs left at the marathon's finish line Monday, which killed three people and wounded more than 180, 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.
And there's more.
The killing of 32 people by gunman Seung-Hui Cho at Virginia Tech University fell on April 16, 2007, while the Columbine school massacre of 13 people happened April 20, 1999.
Doctors say carpentry nails have been taken out of the bodies of victims from the Boston attack. The same nails were used in the pipe bomb that killed a woman during the Atlanta Olympics in 1996.
"But the technology does not really belong to anyone," said J.M. Berger, a consultant and researcher on terrorism.
Nails and other metal shrapnel have also been used in improvised roadside bombs left in Iraq, Afghanistan and other conflict zones.
Investigators say they suspect the Boston bombs were in pressure cookers left in backpacks.
Such bombs have been used in Iraq and Afghanistan, and Al-Qaeda supporters published a guide to making the weapon last month, according to the SITE monitoring group that follows Jihadist websites.
Jeffrey Grossmann, a security specialist at St. John's University in New York, said such attacks "require advanced preparation and dry runs that may have been happening without detection for months".
"The timing of the event (Patriots' Day) and the high-profile Boston Marathon were likely contributing factors in choosing where and when to to act," he added.
Berger, meanwhile, noted that the marathon was "an attractive target if you are a terrorist group that wants to get an identity. But for the moment it is all speculation and eventually someone will be right."
Police and the Federal Bureau of Investigation have appealed for people who were at the marathon to hand over video and still pictures, particularly those taken just before the explosions.
After the Atlanta bombing, police were able to trace the nails found in the dead woman back to the manufacturer as they hunted Eric Rudolph who was eventually found guilty of the attack and is now serving a life jail term.
Alexia Ash, chief North America forecaster for Exclusive Analysis, a specialist intelligence company, said the attacks were "intended to cause mass casualties."
"The scale of the attack, coupled with the fact there has been no claim of responsibility indicates that this was likely the work of an individual or a small group," Ash adding that there was an "elevated" risk of new shooting or bomb attacks.
"The war on terrorism is far from over, whether it is Islamic jihadists or right-wing extremists," said Representative Peter King, a member of the House Intelligence Committee.