The two bombs planted by the brothers killed three people, including an 8-year-old boy, and injured as many as 260 others.
Todashev's charge that his son was intentionally killed by the FBI dovetails in some respects with a conspiracy theory advanced by the father of the two bombing suspects, Anzor Tsarnaev, who said his two sons were "set up" because they were Muslims.
Within American intelligence agencies, the shift toward a view that Mr. Hussein and his sons are probably alive has been prompted in part by the failure of excavations of the two bombing sites to turn up DNA or other physical evidence of their bodies.
The rooms containing the Queen's Gallery are on the site of the former chapel, which was damaged by one of the seven bombs to fall on the palace during World War II.
Tuesday's memorial events began with a wreath-laying ceremony at the site of the explosions, attended by the families of the three bombing victims - Martin Richard, Krystle Campbell and Lu Lingzi.
At Edgware Road, passengers from a train travelling parallel to the one bombed by Mohammad Sidique Khan smashed the windows of their own carriage and clambered across the tracks to hurry to the aid of the wounded "without regard for their personal safety", said Mr Keith.
Later in the day, Nagato was hit in the bow by two bombs, but the damage was not severe.
Haunted by images of the human devastation caused by the two atomic bombs - disconcertingly dubbed 'Little Boy' and 'Fat Man' - dropped on Hiroshima and Nagasaki to end the Second World War, a generation started marching.
At 12 43 Nagato was hit in the bow by two bombs, but the damage was not severe.
