Tribune staff and wire reports

1:25 p.m. CDT, April 17, 2013

Authorities may have taken into custody a suspect in the Boston Marathon bombings based on security video that showed a man depositing a bag at the bomb scene before the blasts, according to two media reports citing U.S. and Boston law enforcement sources.

CNN and Fox News have reported an arrest has been made, while the Associated Press and Boston Globe say the bombing suspect is in custody and expected in federal court. However, Reuters, CBS and NBC are reporting there has not been an arrest in the case.

An official announcement is expected at 4 p.m. central time.

Photos: Remains of Boston Marathon bombs

Bombing investigators have searched through thousands of pieces of evidence from cell phone pictures to shrapnel shards pulled from victims' legs.

Based on shards of metal, fabric, wires and a battery recovered at the scene, the focus turned to whomever may have made bombs in pressure cooker pots and taken them in heavy black nylon bags to the finish line of the world-famous race watched by crowds of spectators.

The twin bombs in Boston, which killed three people and injured 176 others, was the worst attack in the United States since security was stepped up across the country after the September 11, 2001 hijacked plane strikes.

A stretch of Boston's Boylston Street almost a mile long and blocks around it remained closed on Wednesday as investigators searched for clues. The explosions sprayed shrapnel far enough that police were collecting fragments from rooftops along the marathon's course.

Nylon fragments, ball bearings and nails

Among the items recovered at the bomb scene 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, Richard DesLauriers, the Federal Bureau of Investigation's special agent in charge in Boston, told a news conference on Tuesday. Evidence collected at the scene was being reconstructed at the FBI laboratory in Quantico, Virginia, 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.

"That gives you an idea of the scope, of the power of the blast, and you can see why it was so devastating," said Gene Marquez, acting special agent in charge of the ATF Bureau in Boston.

One picture shows a few inches of charred wire attached to a small box, and another depicts a half-inch 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 U.S. government official, who declined to be identified, made the pictures available to Reuters.

Pressure cooker bombs in wide use

Pressure cooker-style bombs are common in South Asia, accounting for roughly half of the explosive devices defused in the country’s volatile northwest, a top Pakistani bomb disposal squad official says.

"We are defusing pressure cooker bombs almost daily," said Shafqat Malik, chief of the bomb disposal squad for Khyber-Pakhtunkhwa province, which includes the violence-wracked city of Peshawar, Swat Valley and Pakistan’s militant-ridden tribal areas along the Afghan border. "They're very common. Pressure cookers are one of the favorite IED containers for the terrorist groups."

Since Malik began leading the province’s bomb squad in 2009, his officers have defused more than 5,000 explosive devices — roughly half of which have been pressure cooker bombs, he said. This year alone, Khyber-Pakhtunkhwa province bomb disposal technicians have defused about 125 bombs that have been contained in pressure cookers, he said.

The most recent prominent attack involving such an explosive occurred Sunday in the Swat Valley town of Banjot. Mukarram Shah, a member of the secular Awami National Party (ANP), was killed when a pressure cooker bomb planted near his car exploded.

Pakistani Taliban militants rely heavily on pressure cooker bombs, Malik said. because they allow pressure to build up inside the steel before the blast occurs, creating a more intense explosion.