(BOSTON) -- Overnight, federal agents and police discovered what could be fresh evidence of how the men believed to be responsible for the twin detonations at the Boston Marathon finish line constructed their bombs.
Agents uncovered a stash of firework shells, some of which appeared to have been emptied of their gunpowder. The cache was discovered in a used clothing collection bin in a grocery store parking lot in Watertown, Mass.
"It was a device that looks like a big firecracker … that had a wick in it," said Michael Tambosi, a representative of the group Planet Aid New England, which owns the clothing donation containers.
A driver for the organization first spotted the fireworks when he opened the bin. There were about half a dozen Roman candle shells in a plastic grocery bag and Tambosi said it looked like some of them had been scooped out.
Investigators responded late Wednesday, loading the bins on a flatbed truck and carrying them away to the staging area the FBI team has set up for gathering evidence in the bombing case.
Already, authorities have determined that most of the homemade bombs were built with store-bought items, including kitchen pressure cookers, nails and the guts of remote control cars. The source of the explosive materials was still being studied.
Officials will be examining the fireworks to see if the hollowed-out shells match those that Tamerlan Tsarnaev is believed to have purchased from a vendor in New Hampshire, or bear any resemblance to the pyrotechnic device that agents said was discovered in Dzhokhar Tsarnaev's college dorm room.
Explosives experts said the powder from the fireworks can serve as a potent ingredient in homemade devices similar to those that exploded along Boylston Street, killing three and wounding more than 250 marathon spectators.
"They can be extremely powerful," New York Police Department bomb technician Kevin Barry told ABC News.
Officials continued Wednesday to try piece together the actions taken by the Tsarnaev brothers in the days and weeks leading up to the marathon blasts. But that effort will no longer be aided by the hospitalized suspect, Dzhokhar Tsarnaev.
Dzhokhar is no longer cooperating with interrogators, authorities said. He was read his Miranda rights on Monday after 16 hours of questioning, which interrogators said was stretched out as long as possible.
Authorities say at this point they don't believe the two had plans for any other attacks. But they also believe there is evidence that the two brothers might have been planning a trip to New York.
The man who was car-jacked by the alleged bombers has reported he may have overheard them talking about Manhattan. Police were also scrutinizing photos posted on a Russian social network page by a friend and one-time classmate of 19-year-old Dzhokhar Tsarnaev. The photo showed Dzhokhar and others in Times Square a year ago.
The New York police commissioner said he had been told by investigators that the two brothers were talking about traveling to New York after the bombings.
"The bit of information that we have ... may have been words to the effect of coming to New York to party," said New York City Police Commissioner Raymond Kelly.
People in Boston continue to struggle to understand the motive of the bombings and the murder of a police officer on the MIT campus. At Wednesday's memorial for the slain officer, Vice President Joe Biden spoke with angry contempt of the Tsarnaev brothers, calling them, "Two twisted, perverted, cowardly, knock-off jihadis here in Boston."
One lingering mystery lies in what changed for Dzhokhar Tsarnaev, who spent most of his childhood and formative years in American schools and was widely described as easy going and well liked.
Just last year, on Sept. 11, in a patriotic ceremony on the floor of Boston's TD Garden, the University of Massachusetts student and casual soccer player sat among 2,500 immigrants and swore his allegiance to the United States, becoming a naturalized citizen of the United States.
Seven months later, authorities say he told FBI agents that he and his brother planted bombs along the route of the revered sporting event and killed a police officer. The reason he gave, according to authorities, was that Muslims are at war with the U.S.
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