Boston Marathon bombing

The Boston Marathon bombing was an attack at the annual Boston Marathon in Boston, Massachusetts on April 15, 2013, carried out by two Chechen-American brothers, Tamerlan and Dzhokhar Tsarnaev. Two homemade explosive devices detonated near the finish-line of the 26.2 mile footrace route, killing three people and injuring 264. During the ensuing manhunt--during which Greater Boston was virtually shut down--Tamerlan Tsarnaev was killed along with one police officer, while 16 other officers were wounded capturing Dzhokhar.

Reactions
Within hours, and without any evidence (sound familiar?), speculation about motives and responsibility started appearing on the internet.

The race began after a minute's silence for the victims of the Sandy Hook massacre. Even this led some to connect the two events initially.
 * Fox News and surrogate New York Post reported that a 20 year-old Saudi man was a suspect, but this turned out to be fictional. The New York Post also reported significantly inflated deaths/casualties, conceivably from thin air.
 * Fox News/World Net Daily contributor Erik Rush blamed Muslims, suggesting that they should all die for this. (To put things into perspective, that's over 1.5 billion people, so 75 times the scale of the Holocaust. Brilliant!)
 * A "report" that CNN's Wolf Blitzer blamed the Tea Party or other tax protestors made the rounds on Twitter. The report turned out to be completely bullshit.
 * The Westboro Baptist Church reacted predictably, blaming the gays and Massachusetts (being the first state in the United States to legalize same sex marriage).
 * The religiously brainwashed made their opinions known on Twitter, blaming atheists and godlessness.
 * NBC's Luke Russert made the connection to the Waco, Texas events of 1993, an idea which was only taken seriously by wingnut pundits as an attempt to paint liberals as liars and thugs.
 * Jordanian Al-Qaeda head Mohammad al-Chalabi also reacted predictably, praising the attack.
 * Reddit's claim that a 22-year-old from Rhode Island (who disappeared a month earlier) was one of the culprits, was propagated around the Internet and considered news. He was found dead on April 23.
 * Alex Jones, as usual, asserted on Infowars that the attack was a false flag operation.
 * Various other conspiracy theorists concurred with Jones. A recurring piece of "evidence" was an episode of Family Guy from the previous month, in which the character Peter Griffin kills several Boston Marathon runners by driving a car over them; some claimed this was predictive programming. Apparently, the programming wasn't predictive enough for certain conspiracy theorists: a doctored video was posted on YouTube combining this sequence with an unrelated scene elsewhere in the episode, showing Peter causing several explosions.
 * said no more visas should be granted to Muslims, and no more mosques should be created. Far-right commentator Bryan Fischer agreed with these views.
 * Some conspiracy theorists inevitably linked the attack with the West Texas explosion that happened two days later.
 * Some of the loopier conspiracy theorists claimed the whole thing was staged, and witnesses and victims (some with horrific injuries) were all "crisis actors" faking it. Stella Tremblay, a New Hampshire state legislator (and all-around moron), argued that it was clear that victim Jeff Bauman was a plant, as he was obviously "not in pain" following the loss of his legs, as traumatic shock is clearly not a thing that exists (it is not explained how no workers at the 100+ hospitals that treated victims came forward and said the injuries were faked). These claims resulted in some pushback by conspiracy theorists who believed the bombing was a false flag attack, but who nevertheless thought the bombing was real.

Date relevance
The date of the bombing had some interesting relevance which fueled speculation. April 15th is the tax filing due date in the United States, so tax protest was considered a possible link (particularly in the same city as one of the first tax protests in the country, the Boston Tea Party, no relation to today's Tea Party).

The date was also the observance day for Patriots' Day (which also is the day chosen for the annual marathon), a Massachusetts state holiday, which technically lands on April 19th, in commemoration of the Battle of Lexington and Concord. April 19th also happens to be the same day as Oklahoma City bombing in 1995, and the end of the Branch Davidians compound siege. As if to bookend the story, April 19th was also the day the last surviving suspect was taken into custody.