The allegations came in a report submitted to a court investigating the public listing of the group, many of whose customers lost their savings.
The financial evaluation of Bankia included in the brochure for the stock listing "did not present a faithful picture of the company", the Bank of Spain experts wrote.
They said they had discovered "inexplicable purchases" of Bankia shares and "sales immediately after the stock listing that cast doubt on the real interest of certain investors".
Madrid's National Court is investigating former IMF head Rodrigo Rato, who was chairman of Bankia at the time, along with other suspects. None of them has yet been formally charged, but they have become targets of popular anger in a country with a 24 percent unemployment rate, fanned by a string of political corruption scandals.
Bankia nearly collapsed in 2012 and had to be bailed out by the Spanish government for €24 billion ($30 billion).
Thousands of customers who say they lost their money after being misled into converting their savings into Bankia shares have brought separate lawsuits against the group. In several hundred cases judges have ruled in their favour, according to associations representing those affected.
Rato is former economy minister of Spain and former head of the International Monetary Fund, the global lender that later played a leading role in tackling the eurozone debt crisis.
The experts' report also alleged accounting failures dating to just after Bankia's current chairman Jose Ignacio Goirigolzarri took the helm. The experts said the group overvalued its real estate assets in its accounts for 2011, damaged by the property crash that hurled Spain into crisis in 2008.
The accounts "do not fulfil the Bank of Spain's norms, due to accounting errors," the experts wrote. These errors "are the result of omissions or imprecisions due to the failure to use the information that was available when the financial reports were drawn up, and which the BFA-Bankia group could and should have used."