The Boston Marathon explosions were caused by pressure cooker bombs, studded with shrapnel, say investigators. FBI officials believe the pressure cookers were hidden inside of backpacks. Among the shrapnel were ball bearings, BB pellets and chunks of metal.

Although investigators are beginning to piece together more information about the devices that caused the Boston Marathon explosions, there are still no suspects in the bombing.

"Someone knows who did this," said Special Agent in Charge Richard DesLaurier at a press conference late Tuesday. "We will go to the ends of the earth to identify the subject or subjects who are responsible for this despicable crime, and we will do everything we can to bring them to justice."

Three people were killed in the Boston Marathon bombing, and between 100 and 130 were injured. According to NBC, 72 patients were still being treated at six Boston hospitals as of 3:30 p.m. Tuesday.

"In 28 years, this is definitely the worst I've seen," said Chief Ron Harrington of the Boston Fire Department's District 3. "Bodies and body parts. Blood all over. A little boy lying in the street. A young woman in her 20s. Both dead. It was mayhem. I saw two people with arms hanging loose, and one without a leg. A shoe with flesh still in it."

While the Boston Marathon explosions were the most devastating act of terrorism on U.S. soil since 9/11, it's not the first time in recent years that a pressure cooker has been used in a bombing attempt. In 2010, an attempted bomb attack on Times Square in New York City also utilized a pressure cooker that was hidden in a parked car. The attempt was foiled when two street vendors alerted an NYPD officer to the suspicious vehicle.