The 11th has now been chosen as an attack date in New York, Madrid and Mumbai.
"There's a lot of symbolism involved when terrorists choose their targets and the dates of their attacks," said Jack Cloonan, a former FBI agent on the bin Laden squad who now works as an ABC News consultant.
"Who knows?" said Cloonan.
Last year's attacks in London was on 7/7.
No group has claimed responsibility for today's attacks, but FBI counter-terrorism officials say it is clearer than ever that the relatively unguarded big city mass transit systems have now become the primary target for terrorists across the world.
The plot disrupted last week in the United States also involved a major commuter train line, the PATH trains, which run under the Hudson River from New Jersey into New York City.
Officials say the people involved in that plot had planned their attack for as close to Sept. 11 as possible.
Last year's subway attacks in London came as the G-8 meeting of world leaders was being held in Scotland. Today's attacks come on the eve of this year's G-8 meeting to be held in Moscow.