Authorities in Boston are looking for those who are behind the bombings. And people from around the world are monitoring their progress.
Do not walk too closely to SUVs parked near Copley Square Hotel on Huntington Avenue. If you do, a dog will lunge, at least from inside a vehicle marked "CAUTION POLICE DOG".
He is held back by thick mesh that covers a window, but his snarl is loud - and menacing.
The dog is part of a K9 unit that is helping with the investigation into the explosions - "the most complex crime scene that we've dealt with in the history of our department," said Ed Davis, a police commissioner for the city of Boston.
He was speaking to a group of journalists who had gathered on Tuesday morning at the Westin hotel. All of them had questions, but there were few answers.
Clues from chaos
Outside, the crime scene looks huge. It is roughly 12 city blocks, according to Mr Davis. Still, it is smaller than before. A day earlier, it was 15 blocks.
The investigators are narrowing their search for evidence.
At first, the authorities were looking for bombs. Now they say there were only two - and they are no longer concerned that there are more.
They have released images that appear to show the remains of a dark coloured backpack and a pressure cooker, as well as a triggering mechanism.
A source close to the investigation said the bombs consisted of explosives placed in 1.6-gallon pressure cookers, one with shards of metal and ball bearings, the other with nails. The bombs were put into black bags and left on the ground, the source told AP news agency.
The authorities are now looking for anything that might have been part of the explosive devices - bits of metal or sharp objects that they might be able to trace back to its owner or owners.
Luckily for them, the site, which is near the finish line of the Boston Marathon, is well documented. "This was probably the most well-photographed areas in the country," Mr Davis said at the press conference.
People took pictures and videotaped their friends and family members as they completed the race. Such a highly-photographed, media-friendly area could make the site a popular pick for a terrorist looking to make a public impact. But the myriad of photos can also provide crucial clues to investigators.
During the Vancouver Riots, for instance, forensic scientists were able to tag criminal activity in crowd-sourced videos and then track those suspects on tape, all while documenting identifying features and clothing.
Doctors say they have found ball bearings and carpenter nails in the bodies of those who were hurt, according to media accounts. These details, however disturbing, are helpful for the investigators.
Investigators can use the material - and even information about them - to put together a more complete picture of the explosive devises and how they were made. At that point, the investigators might be able to find out where the material was purchased - and who bought it.
Rebuilding the attack
They are trying to navigate unfamiliar territory.
"We know 9/11, but what do we know since 9/11 that we would define as a terrorist attack?" says Karen Greenberg, director of Fordham Law School's Center on National Security. "This is a different thing."
Since 9/11, there have been close calls, but nothing like what happened in Boston. In New York in 2010, for example, a man named Faisal Shahzad tried to set off explosives in Times Square but failed.
Bombing expert
Nevertheless, some aspects of the investigation are familiar. The authorities are examining the explosive devise. "You try to look for fingerprints," says Marc Sageman, author of Understanding Terror Networks.
The investigators will also "reverse-engineer" everything, says a former military officer who once specialized in bombings in combat zones.
In this way, the investigators will try to understand the mechanics of the attack.
The former officer, who asks not to be identified, says he has been under-whelmed by the tradecraft of the terrorists. They bungled the job, he says - and had only minimal impact.
"It's tragic and horrible, but from a strategic point of view it just shows that they're idiots," he says.
"It was just crappy planning from the get go," he says.
The fact that the terrorist or terrorists killed three people in a crowded area of Boston offers some information about their planning, experience and ability.
If the bombers had been more sophisticated in their approach, they could have killed many more. In this way, the culprits inflicted a smaller amount of damage than they might have if they had been more efficient in their bomb making and delivery.
Luck needed
Despite all the high-tech advances since 9/11 and earlier bombings, like the Oklahoma City bombing in 1995 and the Olympic Village bombing in 1996, the biggest breaks may come from good old-fashioned human error.
Mr Sageman says many terrorists do not have the sharpest of intellects, which only helps the investigators.
"People are not always that careful," he says, referring to the perpetrators of these crimes.
After the World Trade Center bombing in 1993, for example, investigators found a car registration number on the axel of a vehicle. That lead law enforcement to a car rental shop in New Jersey, where the bomber later returned.
"The guy who rented the car went back and wanted his money back," says Mr Sageman. "A moron."
In this way, investigations get lucky. "Most of the time they make mistakes," says Mr Sageman, referring again to the terrorists. "Or people brag."
The key to an investigation, he says, is not to have set ideas about what will be found.
"You have to allow the investigation to unfold and to go in there without a prejudiced view," he says.
Meanwhile, the police and their dogs are trying to keep the crime scene clear. At the Marriott hotel on Huntington Avenue, two members of a SWAT team walk through a Starbucks.
Wearing helmets and desert boots, they are heavily armed - and seem better suited for Helmand Province than downtown Boston.
Until there are more answers, this is part of the new normal.
Wednesday 17 April 2013
http://www.bbc.co.uk/news/world-us-canada-22178280
