Explosions at the Boston Marathon, potentially ricin-laced letters intercepted en route to the White House and Sen. Roger Wicker, and an explosion at a fertilizer plant in Texas have each prompted investigations. In each case, authorities sift through evidence to construct a timeline of events.

Investigators have said the key clue is likely to come from photos or video taken by the public, and social media sites are buzzing with theories about possible suspects. But with so many images out there, it's like trying to find the one slightly off-white pingpong ball buried under 10,000 white pingpong balls.

Running is big in Boston which is home to one of the world's most famous marathons. Small shops that specialize in running shoes and gear line the route. One shop sits right at the finish line, where Monday's attack killed three people.

The FBI has arrested a Mississippi man believed to be responsible for mailing three letters that, at least preliminarily, tested positive for the poison ricin. They were addressed to President Obama, Senator Roger Wicker of Mississippi and a judge in Mississippi.

The investigation into the Boston Marathon bombing is ongoing. Investigators are examining photos and videos and testing physical evidence recovered from the scene. Without any official news, speculation has been rampant.

The number now stands at 52, the U.S. military says. The news comes just days after guards raided a section of the facility to move prisoners to single cells from their communal holding area because the detainees had covered security cameras.

Pressure cooker bombs have long been used in places such as Afghanistan and Pakistan because they are cheap, easy to build and inconspicuous. They rely on basic principles of physics to amplify their explosive power.