It actually started to dry up after the bombing in OmaghTony...

It actually started to dry up after the bombing in Omagh
Tony Blair also put a lot of pressure on the yanks with a threat not to assist them with their inquiries after a couple of GI's were gunned down in Germany and varies bomb attacks on Embassies.9/11 put the icing on it.

Yanks being yanks as history has proved in 1914-18 and 1939-1946 do nothing until it effects them directly.
Another reason why we shouldnt give to much credance to the idea that they will come running to help us if needed.