Boston Marathon bomb scene pictures taken by investigators show the remains of an explosive device. The photos were produced by the Joint Terrorism Task Force of Boston, provided to Reuters April 16, 2013 by a US government official who declined to be identified.[Photo/Agencies]

BOSTON - Boston Marathon bombing investigators on Wednesday entered the third day of their hunt with an emerging picture of the target: a suspect or suspects carrying heavy bags or backpacks made of dark nylon.

While still unable to conclude whether a group or individuals were responsible for the attacks that killed three people and wounded 176, and whether they were foreign or American, investigators gathered enough evidence at the crime scene on Tuesday to slightly narrow their search.

The two blasts struck seconds apart on Monday at the finish line of the race, maiming victims with shrapnel-packed bombs that investigators suspect were contained in pressure cookers. Seventeen people remained in critical condition.

President Barack Obama, who will travel to Boston on Thursday for a memorial service, has called the bombings an "act of terror." It was the worst bombings on US soil since security was stepped up following the suicide hijack attacks of September 11, 2001.

No suspects were in custody and there were no claims of responsibility.

Evidence collected at the scene was being reconstructed at the FBI laboratory in Quantico, Virginia, said Richard DesLauriers, the Federal Bureau of Investigation's special agent in charge in Boston.

Among the items recovered were pieces of black nylon that could be from a backpack, fragments of ball bearings and nails, and possibly the remains of a pressure cooker device, DesLauriers said.

Bomb scene pictures produced by the Boston Joint Terrorism Task Force and released on Tuesday show the remains of an explosive device including twisted pieces of a metal container, wires, a battery and what appears to be a small circuit board.

One picture shows a few inches of charred wire attached to a small box, and another depicts a half-inch (1.3 cm) nail and a zipper head stained with blood. Another shows a Tenergy-brand battery attached to black and red wires through a broken plastic cap. Several photos show a twisted metal lid with bolts.

A US government official, who declined to be identified, made the pictures available to Reuters.

In addition, Boston's WHDH television showed a picture of an unattended, light-colored bag on the ground right at one of the bomb sites before the explosion. The bag was gone in a picture from a similar angle taken after the blasts. Authorities had yet to comment publicly on the significance of the pictures. ?

The youngest to die was an 8-year-old boy, Martin Richard, from the city's Dorchester neighborhood.

Officials identified a second person killed as Krystle Campbell, 29, of Medford, Massachusetts.

The third fatal victim was a Chinese citizen whose identity was not being made public at the request of the victim's family, the Chinese Consulate in New York said in a statement. The victim was a graduate student at Boston University, the university said in a statement.