CSI Boston: Painstakingly Searching One of the Largest Crime Scenes in Recent History

Inch by inch, authorities look for evidence they hope will one day be used to forever put away those responsible for the Boston marathon bombings

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Elise Amendola / AP

One of the blast sites on Boylston Street near the finish line of the Boston Marathon is investigated by two people in protective suits in the wake of two blasts in Boston on April 15, 2013.

On busy Newbury Street in Boston’s Back Bay, tourists and students window shop or grab frozen yogurt. Bostonians walk their dogs and run errands, sometimes pausing to enjoy the first good week of spring weather of the year. A block away on Boylston Street the scene couldn’t be more different. Bomb technicians in white jumpsuits, looking like they’ve jumped out of ET, slowly scour the street looking for bomb debris and evidence.

Investigators are conducting what they call a grid search where every square foot is numbered and documented for evidence. The first thing they did after securing the scene is bring through explosive detection dogs, who alert where they smell traces of explosive so techs know what areas have the greatest potential for evidence collection, Steven Batholomew, a special agent at the Bureau of Alcohol, Tobacco and Firearms, tells TIME. They then determine the blast seats, where the bombs were originally placed, and from there track out the blast field. Though Batholomew didn’t want to specify how large these particular fields are, judging by the rooftop searches the techs were seen conducting Thursday on a six-story building 100 feet away from the first blast sight, it’s fair to say hundreds of feet.

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The techs will scour everything in the blast field for evidence, from rooftops to sewer drains. They will pry out shrapnel embedded in buildings and pavement. They are looking for whatever held the bomb, in this case two black nylon bags; the bomb casing, in at least one case a pressure cooker – not enough of the second casing has been recovered to definitively say if it was also a pressure cooker; shrapnel – in this case ball bearings, nails that had had their heads sawed off and BBs; batteries, switches, timers, wires and circuit boards. All of this gets sent to the FBI’s Quantico laboratory where they reconstruct the bombs, which were detonated 12 seconds and 100 yards apart at the height of the marathon, killing three and wounding 176.

At least 30 ATF agents and hundreds of FBI agents are involved, ranging from special agent bomb technicians – the primary investigators – to explosive enforcement officers, who guide other technicians to patches debris to look for the bomb pieces, to certified explosive specialists. There’s even a chemist on scene who swabs evidence to determine the make up of the explosives used and disposes of any unconsumed explosives. They work in 12-hour shifts and have taken over what was once the marathon’s medical tent in Copley Square.

The Boston scene is unique in that it involves such a huge swath of a busy urban area. The World Trade Center was surround mostly by other office buildings. The Alfred P. Murrah federal building in Oklahoma City was nestled amongst municipal buildings. And the Atlanta bombing of the Olympics affected only a relatively small part of the Olympic Park. Boylston Street, on the other hand, is one of the busiest shopping streets in the world. It is the Boston equivalent of bombing Rockefeller Center in New York and shutting down a 10-block stretch of Fifth Avenue for the better part of a week.

“We are securing and processing the most complex crime scene in the history of our department,” Boston Police Commission Ed Davis said on Tuesday, begging the public’s patience with the closure of such a major zone. The scene has shrunken from 15 square blocks in the immediate aftermath of the bombings to just eight blocks on Thursday. The Boston Police said on its twitter feed the scene will remain closed at least until Friday.

In many ways, the lovely weather has been a boon for the bomb techs searching the crime scene. There was some worry that forecast rain could impede the investigation. “Rain could impact residue,” Batholomew told reporters on Tuesday. “There may be smaller components where water could wash it away but I don’t want to say specifically.” Though it has rained in other parts of Boston, Boylston Street has remained under sunny skies.

For many Bostonians peering at the investigation from the sidelines, this may not be the last time they see these FBI and ATF technicians. “These are the experts that will testify at any subsequent criminal proceeding,” Batholomew says. Because every tiny BB pried out of a concrete wall or taken from the leg of a victim is a piece of evidence that could lead to –and incriminate – the person or persons responsible.