Possibly significant, not because of what he said but because there’s a roommate at all. If you’re building bombs, the person you live with is likeliest to know, yet I haven’t seen anything suggesting that the roommate’s under suspicion. Evidently the FBI didn’t haul him in last night for questioning. Assuming he’s innocent, then either the man in the hospital’s innocent too or he was probably building the bombs somewhere else. Where? With whom?

Or maybe, despite the damage done, the bombs were small enough that they could have been made in a single room without someone nearby knowing. Anyone with military/LE experience know how much space someone would need to build a pipe bomb? The device itself is small, I know, but do you need some clearance maybe for ventilation (or other reasons)?

The explosions at the Boston Marathon on Monday were likely caused by high-power black powder explosives, bomb experts tell U.S. News, and at least one of them was likely shaped in a pipe bomb.

Footage of the initial detonation of the blast showed an explosive plume that would align with a high-powered pipe bomb, says Fred Burton, a retired Diplomatic Security Service special agent who helped investigate the first World Trade Center bombing, among other high-profile attacks…

Building a pipe bomb does not require a great degree of sophistication, Burton says, but getting them to function properly “does take some practice.”…

The white smoke that emanated from the blasts indicate this was likely a smokeless or black powder, he says, not a military-type explosive such as C-4 or plastic explosives, which give off black smoke.

Where did the bomber end up practicing? Did he learn from scratch as a lone wolf here in the U.S., setting off test bombs somewhere in the woods, or was he trained elsewhere so that he didn’t have to test them in Boston? Again, don’t assume that it’s the man in the hospital: “Officials cautioned that the man may turn out to have been an innocent bystander and said it was too early to know.”

The New York Post has a little more about him:

At the hospital, investigators seized the man’s clothes to examine whether they held any evidence that he was behind the attack. The law-enforcement sources also told The Post that the man was not free to leave the medical center…

He also said that he went to the Copley Square area yesterday to witness the finish of the race.

The sources said that, after the man was grabbed by police, he smelled of gunpowder and declared, “I thought there would be a second bomb.”

He also asked: “Did anyone die?”

The “second bomb” line is interesting, but on 9/11 I was waiting for a second explosion too. When I saw the damage from the plane to the North Tower, I got a silly idea in my head that somehow the burning wreckage would hit a gas line in the building and that would cause another boom. Frankly, at this point, it’s a solid prediction during terror attacks that a second bomb will follow the first. Did anyone else there smell of gunpowder, though? I’d imagine that would be common among the injured in the vicinity of the blast but military/LE readers are welcome to correct me. At the very least, I still don’t understand why this guy, if he’s the bomber, would have been close enough to the explosion to sustain injuries from it. If he wanted to admire his handiwork, he didn’t need to be there for that. He had to have known that there were cameras at the finish line of the Boston Marathon that’d capture it. And even if he insisted on being there, he wouldn’t stand so close as to put himself in danger. Hard to explain that behavior if he’s the bomber, which means he might not be.

If you missed it this morning, the cops have issued an alert for a rental van that might have been trying to gain access to the finish-line area yesterday. That’s hair-raising, but presumably also innocent: If the bomber(s) had a van packed with explosives ready to go, he would have set it off even after being denied access. Unless the bomb fizzled, of course, which has been known to happen. Exit question for criminal-law lawyers: How could the feds have gained access to the apartment of the man in the hospital? Either the man himself consented, suggesting he was unconcerned with what they’d find there; they had probable cause, which seems unlikely given that the cops refuse to even call him a suspect; or the roommate could have consented, right? If the roommate consents, can the feds search the entire apartment or only the areas common to both roommates, i.e. not the bedroom of the man in the hospital?

Update: Back to square one?

The day after two powerful bombs exploded near the finish line of the Boston Marathon, a mile-square area around Copley Square here remained cordoned off as a crime scene, and officials still had no one in custody. Investigators searched a house in a nearby suburb late Monday night, but later said the search had proved fruitless…

Late Monday night, law enforcement officials descended on an apartment building in the suburb of Revere, about five miles north of Copley Square, linked to a man the police took into custody near the scene of the bombings. But on Tuesday morning, one law enforcement official said investigators had determined that the man, who was injured in the blast and was questioned at the hospital, was not involved in the attack.

GOP Rep. Michael McCaul told the Times that the device appears to be a “pressure-cooker”-type IED of the sort used in Iraq and Afghanistan. Also:

Pressure cooker is used in a number of bomb recipes, but notably in Inspire, so we're going to hear a lot of speculation on the front now.

— 𝗝.𝗠. 𝗕𝗘𝗥𝗚𝗘𝗥 #readoptimal (@intelwire) April 16, 2013

“Inspire” is the Al Qaeda “magazine” produced by the group’s Yemeni affiliate.