I don't see why anyone expects to see the bomber after the bomb--
My theory--the bombs were in the trash can the night before. Why did so many people get their legs full of shrapnel? That suggests to me that the explosion happened close to the ground, where legs and small children are. People have been at the corner all day, throwing trash on top of the bomb. Accumulation of trash and position of the bomb suggest to me that it was there from the start of the day.
I'd like to know when those cans were placed, when they were last cleaned. Were they brought in special for the run? Can bomb-sniffing dogs smell one of these bombs made in a pressure cooker? (I guess the pressure cooker, which has a strong seal...you know, I don't know why a pressure cooker makes a bomb more than another pot would...)
How many devices in total did they find?
Placing the bomb inside the building increases the lethality of the device using the glass from the store windows vs. large crowd of pedestrians.
One of the news reports I heard (take w/ grain of salt) said that the bombs would likely have been placed w/in an hour of the attack. I wondered how they would know that and then a few minutes later they followed up with that the area had been swept for explosives about an hour before the explosions (I think with explosive detecting k-9s).
I was fortunate to see a demo of a k-9 dog trained to do explosive detection. These dogs are amazing and the training is amazing.
The handler asked someone with a large purse to volunteer, and who wasn’t going to need to go through airport security for some time (I want to say it was a month or so). He put a piece of cable in her purse (looked like co-ax, but it was a kind used to detonate explosive devices.) The dog went through the room and alerted to the cable. The reason the person couldn’t be going through airport security any time soon, is that dogs and/or electronic “sniffers” could still pick that up for the length of time specified. I wish I could recall the exact amount of time, but the demo was over a year ago.
The handler also had hidden other types of explosives around the room before bringing the dog in. They keep the explosives wrapped and handle only with gloves, so as not to get their scent on the samples. They have to get fresh samples periodically.
They do not cross-train drug sniffing dogs and explosives sniffing dogs— kind of a comical point, but if they alert, it’s like, “oh, great, is it drugs or bombs?” So, the ones for explosives are dedicated to explosives, drug dogs
They continue training, continue certifying/re-certifying. Drug sniffing dogs when tested can have up to (IIRC) 2 missed alerts. The thought being that if the dog misses a baggie of pot, it’s not that big a deal in the grand scheme of things. Bomb sniffing dogs cannot miss an alert.
If the team gets a call for a suspicious package, they go and investigate. The handler told of a case where another police agency called and said they had a bomb and would they bring the k-9 out for it. The handler said, “No, you’ve already identified it as a bomb.” At that stage, they need to call in the bomb squad.
They do train also for tracking suspects. I’ve worked with training dogs for agility and the type of training done for that is mostly by commission. Do x, do y, do z. Some of the training for k-9s is by omission. They can have put 4 people in a car, then have 3 present and one missing (hidden). They have the dog scent the 3 people who are left, and the dog has been trained and knows to search the scents of the one (or more than one, depending) who isn’t present.
Based on the demonstration and explanation of how things work, I believe that yes, bomb-sniffing dogs would be able to detect explosives in a pressure cooker. Likely, the person making it would have left residue around it. But even if if they were extremely careful about that, I believe that the dogs would still have alerted, despite it being sealed.