Not a pipe bomb after all, per one source who spoke to the AP. The explosives were reportedly placed in a pressure cooker, which in turn was placed in a backpack. And they came with accessories:

The metal fragments found in marathon-goers — 20 pieces, or 30 or more in some people — are too uniform, Dr. George Velmahos, trauma chief at Massachusetts General Hospital said during a morning press conference. They look like pellets or nails, something meant to do harm.

“My opinion is that most of them were in the bomb,” Velmahos said. “I think it’s unlikely they would be so consistent if they were pulled out from the environment.”…

Surgeons at Mass. General removed the lower limbs of several patients, some of whom came in so injured that they were considered “almost automatic amputees,” Velmahos said.

In those cases, he said, “we finished what the bomb started.”

What’s a “pressure-cooker IED”? The best summary I’ve found in googling is this undated bulletin (PDF) from Homeland Security posted on a Coast Guard website:

The Department of Homeland Security is issuing this information bulletin to alert frontline border inspectors and agents, state and local officers, and other first responders that there is continued interest by terrorist organizations touse innocuous items to package improvised explosive devices (IEDs.) A technique commonly taught in Afghan terrorist training camps is the use/conversion of pressure cookers into IEDs…

Typically, these bombs are made by placing TNT or other explosives in a pressure cooker and attaching a blasting cap at the top of the pressure cooker. The size of the blast depends on the size of the pressure cooker and the amount of explosive placed inside. Pressure cooker bombs are made with readily available materials and can be as simple or as complex as the builder decides. These types of devices can be initiated using simple electronic components including, but not limited to, digital watches, garage door openers, cell phones or pagers. As a common cooking utensil, the pressure cooker is often overlooked when searching vehicles, residences or merchandise crossing the U.S. Borders.

Terrorists have been using them for more than a decade, most frequently in Nepal, Afghanistan, India, and Pakistan. They were used in the 2006 Mumbai attack, and a very crude version was part of the car-bomb attempt in Times Square in 2010. Michael Yon wrote about them just a week ago and explained why they’re so common to that region:

In Nepal, Pakistan, and India, pressure cooker bombs have been common. The pressure cookers can make powerful IEDs.

Many Nepalese (during my 13 trips there, I have gotten an earful) were angry because police and Army would seize their pressure cookers and not return them.

This might sound strange or inconsequential from afar, but for people who live at high altitudes with little fuel, pressure cookers are as important as running water. It is difficult to cook well and cheaply without a pressure cooker.

Pressure cookers are ubiquitous over there so they don’t arouse suspicion. Al Qaeda’s Yemeni-based “magazine,” “Inspire,” has also recommended them to budding jihadists. Via Ace, the Jawa Report noted this passage from Inspire after Naser Jason Abdo’s bomb plot against Fort Hood was foiled:

The explosion that results from this device is a mechanical one. It results from the pressure caused by the gases and therefore it only works if contained in a high pressure environment. So you may use iron pipes, pressure cookers, fire extinguishers, or empty propane canisters….

The pressurized cooker is the most effective method.

So essentially it’s a larger version of a pipe bomb, with the cooker’s tight seal helping to magnify the force of the blast. Per the Jawas, a pressure cooker, shrapnel, and battery-operated clocks were found in Abdo’s hotel room; per the latest from Boston, the bomber yesterday likely used timers, not cell phones, to detonate the bombs.

I wonder how many FBI agents are tasked right now with gathering the records of all pressure cookers sold in the Boston area over the past, say, six months. And I wonder if the bomber was dumb enough to buy two in the same purchase, which might stick in a clerk’s memory, or if he bought them individually.

Update: Little more from Fox News:

The source, who spoke on condition of anonymity, said a pressure cooker was attached to a wooden board in at least one of the blasts. The pressure cooker acted as the timer, the source said, and attached to the board was a bottle filled with nails, ball bearings and BB’s. That device was placed inside a black nylon backpack and then dumped in a garbage can, according to the source.

A third source said that the FBI is scanning cellphone tower records for the “moment of detonation.” If the moment of detonation is positively identified, the corresponding cellphone number can be traced.