UPDATE: Boston Marathon Bombing

With the finish area secured and casualties being treated, Boston dives into an investigation

Yesterday, two small, powerful bombs exploded near the finish line of the 117th Boston Marathon, transforming what began as a fantastic day of dramatic racing into a chaotic scene of panic and, eventually, despair.

According to the Boston Police Department, at least 176 people were injured—17 of them critically—and three were killed, including 8-year-old Martin Richard, of Dorchester, Massachusetts.  Nobody has yet claimed responsibility for what authorities are treating as a terrorist attack. 

In the initial chaos following the two explosions, which both occurred on Boylston Street around 2:50pm Eastern, officials reported that additional explosives and suspicious packages had been found, some of which police detonated as a safety precaution. Today, however, Massachusetts Gov. Deval Patrick said that there were only the two bombs.

A young Saudi national here on a student visa was questioned as a suspect witness (updated at 5pm), and his suburban Boston apartment searched, but so far the investigation has turned up no further leads.

CNN obtained a law enforcement advisory Monday telling police to be on the lookout for a "darker-skinned or black male," possibly with a foreign accent, who was seen with a black backpack and sweatshirt trying to get into a restricted area about five minutes before the first explosion.

The Boston Athletic Association, organizer of the Marathon, said last night that “all of the remaining runners who were out on the course when the tragic events unfolded have been returned to a community meeting area.”

We’ll update this story throughout the day as critical information becomes available.

5:13PM EDT—A second victim was identified today as Krystle Campbell, 29, from Medford, Massachusetts.

Also, the Associated Press reported this afternoon that the bombs “were fashioned out of pressure cookers and packed with shards of metal, nails and ball bearings,” similar to explosives that have been used before in Afghanistan, India, Nepal and Pakistan:

A person who spoke on the condition of anonymity because the investigation was still going on said that the explosives were put in 6-liter pressure cookers, placed in black duffel bags and left on the ground. They were packed with shrapnel to inflict maximum carnage, the person said.

The person said law enforcement officials have some of the bomb components but do not yet know what was used to set off the explosives.