Dzhokhar Tsarnaev did not plant the pressure cooker bomb at the Forum restaurant. His bag is well pictured in the famous video footage of the corner Gloucester/Boylston Street. It does not contain a pressure cooker (no bulge), it doesn't fit the pictures of the remnants of the second bomb backpack, it doesn't fit the FBI's verbal description of the backpack, and Dzhokhar didn't place it at the location where the second bomb exploded (it was 12-15 feet away, to be exact).
Notwithstanding, his behavior at the Marathon is a conundrum. He dropped his backpack in front of the Forum, and unless he "forgot" to pick it up when he left the site - which is highly unlikely - the question what was in his bag and why did he leave it there is extremely puzzling.
It is not disputed that Dzhokhar purchased and played with fireworks, opened the tubes and removed the black powder. But the producer has ensured that this black powder has not the explosive force necessary for a pressure cooker bomb.
It is also not disputed that there were some bomb alerts after the bombings. The Mandarin hotel was evacuated due to a bomb threat, and there was one controlled demolition announced one hour after the bombs, located across Boston Public library, which is exactly the first bomb site!
Reports from social media (unconfirmed) indicate that a secondary explosive device has been located outside of Boston's Mandarin Oriental Hotel and will be destroyed (hopefully safely) in a controlled demolition by the Boston Police Department using explosive ordnance disposal equipment.
A third explosion was heard about an hour after the first two after authorities warned spectators to expect a loud noise from a water cannon that police apparently were using to destroy one of the devices.
The media have taken the information about the additional suspicious items with astonishing serenity, not to say negligence. It soon vanished into oblivion. Nobody cared about the nature of these bombs, who might have placed them, etc. - but they should have appeared in FBI records, court filings and investigative reports. They didn't. After Marathon day, nobody was interested in the fact that in addition to the two big bombs two other bombs have been dismantled.
And who found the explosive devices among the many bags and backpacks left back or forgotten in panic? Yes - the bomb-sniffing dogs who were already on scene before, apparently as participants of a drill. A study from the Harvard Kennedy school released in April reports that at least one of the not-explosive bombs was identified by a dog.
This leads to an alternative explanation for Dzhokhar's behavior which is much better in accordance with reality: his bag might simply have contained the harmless black powder from the fireworks, with certain ingrediences similar to the "real stuff" so that the "canines" were able to identify it. He was definitely not the only one who placed a harmless bomb as part of the bomb-sniffing drill. And his brother Tamerlan, the one with the special law enforcement connections, persuaded him to do this dubious, but presumably well paid "job".
When besieged by the FBI in his hospital bed, Dzhokhar indeed seems to have mentioned a bomb, but it is unclear what kind of "bomb" . The crucial details are blacked out in this court document - with the exception of the black powder:
Even the defense admits the "bomb" in Doc 295:
Despite fighting for the admissibility of these statements the prosecution has announced not to make use of this "confession". Is it because Dzhokhar's description is not compatible with a pressure cooker bomb?