Single Gun Recovered From Accused Bombers Younger alleged bomber may have been unarmed in boat during standoff.

April 24, 2013 -- Even though the Tsarnaev brothers made a violent dash from police, allegedly tossing homemade bombs as they fled, police told ABC News they so far have recovered only a single, semi-automatic handgun from the two men now accused of carrying off the Boston Marathon bombing.

The gun, presumably wielded by the older of the two brothers, Tamerlan Tsarnaev, was recovered by police at the shoot-out scene in Watertown, Mass., and is now with the Massachusetts State Police. Sources told ABC News that no guns were recovered from the boat where 19-year-old Dzhokhar Tsarnaev was eventually apprehended after a stand-off with police.

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Law enforcement sources told ABC News the gun recovered from the scene of the Tsarnaev brothers' shoot out with police was a Ruger 9 mm semi-automatic handgun. Sources said the gun is in the custody of the Massachusetts State Police lab and that the serial number on the firearm was obliterated.

Investigators have techniques to raise serial numbers after they have been obliterated so they can be read. Investigators would use the serial numbers to conduct urgent traces on the gun to determine where and how it was purchased.

The Tsarnaev brothers' apparent lack of firearms has led some investigators to theorize that the attack on MIT police officer Sean Collier, in which they allegedly ambushed the officer and shot him five times in the head, was an attempt to arm themselves, officials said.

More details continued to emerge about the violent events that unfolded after the FBI released photos of the men they believed planted the two bombs, killing three and injuring more than 250 marathon spectators, and about the victims of the attack.

Recent figures indicate that 34 people remain hospitalized, with one victim in critical condition.

Also Tuesday, a U.S. official confirmed that Tamerlan Tsarnaev's name was placed on the National Counter Terrorism Center's database, as first reported by the Washington Post. The official said having a name being placed in the overall database does not mean the person is considered a threat. Agencies can nominate individuals to go into the database for varying reasons.

Law enforcement officials told ABC News that it now appears that Tamerlan Tsarnaev began gathering materials for the explosives at least two months ago, when he was spotted at a New Hampshire fireworks store. The store told the FBI that it sold $400-worth of fireworks in February to the accused Boston bomber.

The gunpowder in fireworks is often used in bombs.

"He just wanted the biggest, loudest stuff we had in the store," said Megan Kearns, the assistant manager of Phantom Fireworks, in an interview with ABC News affiliate WMUR.

Kearns said Tsarnaev -- the older of the two brothers now accused in the blast -- bought two large reloadable mortar kits during a two-for-one sale. She said she remembered Tamerlan because of his Russian accent. The store has since confirmed his purchase with store records.

"Pretty much the only thing that was remarkable about him was that he had a Russian accent, which we don't get too many people in here who have Russian accents," she said.

The amount of gunpowder that could be harvested from the kits -- less than half a pound -- would not have been enough to detonate the Boston bombs, Phantom Fireworks VP William Weimer said.

Fireworks have often been used by terrorists to power their bombs, including by the man who tried but failed to detonate a car bomb in Times Square. He bought fireworks from a different Phantom Fireworks store, in Pennsylvania.

"Fireworks will give you what you need in terms of blast," said Kevin Barry, retired detective first grade, NYPD Bomb Squad.

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Barry told ABC News that while the ingredients are easily purchased -- the actual assembly instructions are still hard to find and follow.

"What it seems they did was purchased commercially manufactured mortars, ripped them down and ripped out the powder," he said. "When you confine this powder in a pressure cooker it is very powerful."

The charging document filed in the case against 19-year-old Dzhokhar Tsarnaev also notes that FBI agents found "a large pyrotechnic" in his college dorm room.

Federal agents have analyzed the two pressure cooker bombs used in the Boston Marathon attack and confirmed early speculation that the other components were also built largely, if not entirely of commercially available items, including parts of a remote control toy car, BBs and small nails.

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The analysis was circulated to law enforcement agencies Tuesday evening in a federal law enforcement joint intelligence bulletin.

The report notes that a similar device was found in the thwarted 2010 attempt to set off explosives in Times Square.

"Terrorists can exploit the innocuous appearance and transportability of pressure cookers to conceal IED components," the bulletin says.