Investigators believe Boston bombs were pressure cookers hidden in backpacks

April 16, 2013 Updated Apr 16, 2013 at 4:19 PM CDT

BOSTON, Mass. -- There was an awkward, ominous silence in the center of Boston Tuesday as military vehicles took the place of thousands who had packed the marathon route one day before.

One day after two bomb blasts killed three people and injured more than 170 others, Boston is on high alert.

"Everyone should expect continued heightened police presence and everyone should be vigilant," said Massachusetts Governor Deval Patrick.

Investigators have uncovered new information about the blasts.

The bombs were crafted out of pressure cookers rigged with explosives and packed with shrapnel, then stuffed into backpacks or duffel bags.

"The other objects are small head carpenter type nails that approximately half an inch in length and small diameter," said Dr. Ron Walls, who treated patients at Brigham and Women's Hospital.

Victims described the effects.

"I look at my wife and I see she was hit with something and shrapnel had hit her leg and she was bleeding," said Nicholas Yanie.

The attack is now officially characterized as an act of terrorism.

"Our mission is clear, bring to justice those responsible for the marathon bombing," said the FBI's Richard DesLauriers.

Justice for more than 170 injured and and at least three killed, including Martin Richard, 8, whose mom and sister suffered serious injuries in the attack.

There is a prayer vigil for the Martin family Tuesday night at a local church and tomorrow the city will host an interfaith service to honor and remember all of the victims in the attack.