Both groups claimed responsibility for the earlier bombings.
Officials say the letter includes previously unreleased information about an earlier bombing at a lumber mill.
More than 150 people were injured in the earlier bombing on March 4.
An earlier bombing was on July 26, 1944.
This is when it was realized that the earlier bombings had been carried out by loyalists.
Many earlier bombings were aimed at security forces headquarters.
The earlier bombing of Tokyo by incendiary weapons killed some 100,000 people.
The statement did not provide a location for the earlier bombing, and it gave no details about whether people were killed or wounded in the attacks.
He had offered to help in the cleanup of the earlier bombing but had been met with firm smiles of denial.
The attack comes weeks after the police had killed a young Algerian immigrant who they said was behind the early bombings.