"Inspire magazine is well publicized by the press," says Bill Roggio, a reporter for The Long War Journal, of the al-Qaida publication that instructs readers on how to perform homegrown terrorism. "This is what [al-Qaida] advises to do: Build a bomb like this."
Pressure cooker bombs have long been used in attacks linked to jihadist groups, but also among U.S. bombers - Army Pfc. Naser Jason Abdo assigned to Fort Hood was arrested in 2011 for buying the ingredients to build such a bomb in Killeen, Texas.
Al-Qaida in the Arabian Peninsula has developed a manual on building this kind of bomb, which the FBI publicized in a bulletin warning of its proliferation and danger.
All of this public information makes it easier for copycats to get the training and resources they need to carry out an attack.
All that can be inferred so far is that the bomber knew what he was doing.
"If you're smart enough to pull off an attack like this, you're probably smart enough to cover your tracks," says Roggio.
But another former investigator cautions officials should keep an open mind on how the Boston bombing was planned and executed.
"It's important not to have a preconceived mindset," says Fred Burton, a former Diplomatic Security service who worked the 1993 World Trade Center bombing and 1996 Atlanta Olympics. "Every time we identify what we think is the leading suspect, it turns out to be a 180-degree difference."
Early on, federal agencies such as the FBI believed the Oklahoma City bombing was the work of Hezbollah, he says, but it turned out to be Timothy McVeigh.
"It's very important to take into account when you look at the forensics with this kind of IED, that the range of potential actors that could have put this together really spans the spectrum," Burton says. Separate people or groups may be responsible for paying for the bombs ingredients, for building it and for placing it near the marathon finish line, he says.
"These kinds of devices, they've been around forever going back even to Vietnam," he says. "That's a whole range of different kinds of skill sets, and actors that could have put this together."