BOSTON - Investigators are piecing together elements of Monday's bombings along the route of the Boston Marathon.

Two bombs blew up about 10 seconds and around 100 yards apart near the finish line, leaving the streets stained with blood and strewn with broken glass. The FBI has confirmed that pressure cookers may have been used to make the bombs. FBI special agent in charge Richard DesLauriers says pieces of black nylon and fragments of ball bearings and nails were found. Authorities believe the bombs were placed in a dark-colored backpack or bag.

Authorities have confirmed the deaths of at least three people and have named two of them - 29-year-old Krystle Campbell and 8-year-old Martin Richard. Campbell, a graduate of Medford High School in Massachusetts, had gone with her best friend to take a picture of the friend's boyfriend crossing the finish line, family members said. Martin, on the other hand, was waiting at the finish line for family friends to finish the race. The third victim was identified only as a graduate student from China studying at Boston University.

Of the 183 injured, at least 23 received critical injuries, at least 40 received serious injuries and at least 9 are children, according to CNN.

President Obama addressed the nation from the White House Monday afternoon, promising that the parties responsible "will feel the full weight of justice." Today, he added that the attack "was a heinous and cowardly act" of terrorism. However, the president also said that investigators do not know if the bombings were carried out by an international organization, a domestic group or a malevolent and lone suspect; especially since no one has yet to claim responsibility. Gov. Deval Patrick says President Obama will be attending an interfaith service Thursday in Boston.

Homeland Security Secretary Janet Napolitano said there was no evidence that the bombings were part of a wider plot. But she said national security has been stepped up as a precaution.

FBI agents said they've received "voluminous tips" and were interviewing witnesses and analyzing the crime scene. At a news conference, police and federal officials repeatedly appealed for any video, audio and photos taken by marathon spectators, even images that people might not think are significant. Boston Police Commissioner Edward Davis said investigators also gathered a large number of surveillance tapes from businesses in the area and intend to go through the videos frame by frame. Moreover, Davis said that two security sweeps of the race route had been conducted before the blasts.

About 23,000 runners participated in this year's Boston Marathon, which is traditionally held on Patriots' Day. Celebrated on the third Monday in April, the holiday commemorates the opening shots of the American Revolution back in 1775.

AP wire services contributed to this report.