Two bombs detonated near the finish line of the Boston Marathon on Monday, killing at least three people. At least 176 were injured. So far, authorities have not said if they have found who was responsible:

Richard DesLauriers, special agent in charge of the FBI office in Boston, said the investigation is being led by a joint terrorism task force of federal, state and local authorities. “It is a criminal investigation that is a potential terrorist investigation,” he said at an evening news conference.

News reports in Boston said a surveillance photo showed a man with two backpacks at the scene shortly before the explosions. Boston Police Commissioner Ed Davis said Monday night that there were no suspects, but that the police were talking to some people who may have relevant information. (Read the rest of the article here.)

Witnesses described what they saw:

“It looked as though a building near the finish line had blown up,” said [Kara Zech] Thelen, who stared as a wall of fire, glass and debris erupted from a storefront a few dozen yards away. She saw a huge cloud of smoke, oddly greenish-brown, billowing from the shattered building and then: nothing.

Mapping the hunt for Boston Marathon suspects

“It was pure silence,” she said.

A second later, the posh downtown square was transformed into a scene like something from central Baghdad, with panicked runners and onlookers fleeing amid screams from scores of wounded, past blood-splattered sidewalks strewn with body parts. Even in the confusion, many grasped instinctively what had happened. (Read more from the scene here.)

All of the members of the Montgomery County Road Runners, a local running group participating in the marathon, were reported safe. Meanwhile, police agencies in the Washington region put additional security measures in place as a precaution. The attack is the first major bombing in the United States since Sept. 11, 2001, although law enforcement has disrupted many similar plots in the 12 years since then:

From the FBI to local police departments, law enforcement agencies have dramatically shifted their emphasis to counterterrorism over the past decade, gathering intelligence on both domestic and foreign extremist groups. The George W. Bush and Obama administrations have created an enormous global apparatus designed to track and target terrorists.

But officials have always warned that the United States cannot prevent every attempted strike on U.S. soil. In some recent plots, authorities have benefited as much from luck as investigative skill. (Read the full analysis here.)

An experienced race official told Mike Wise, sports columnist for the Post, that securing a marathon is always difficult:

“The one opening in the security planning for everybody were the fans; there is no way to screen your fans,” said the official, who spoke on condition of anonymity. “You can screen your runners, volunteers, media and race officials — you can make them all produce proof that they are who they say they are — but if the arena of competition is a city’s streets, how do you possibly screen every person walking up to the race?”