The former Spanish prime minister has admitted he may have been distracted from the threat posed by Islamic militants by his fight against Eta.
Mr Aznar was voted out three days after the attacks
Jose Maria Aznar's party was ejected from power only days after Islamists blew up trains in Madrid, killing 191.
In a new book he writes that his government may have lowered its guard before the attacks on 11 March.
Mr Aznar originally blamed the blasts on the Basque Eta group, and voters seem to have suspected a cover-up.
Some voters believed the bomb attacks were linked to Spain's support for military action in Iraq.
The country's new Prime Minister Jose Luis Rodriguez Zapatero promised soon after being elected to withdraw Spanish troops from Iraq.
Mr Aznar said Eta and Islamist groups should both be rejected, in his book on his time in power, "Eight years of government, a personal view of Spain".
"I must acknowledge, however, that Spanish public opinion was perhaps not sufficiently aware, until 11 March, of the extent of the threat of Islamic terrorism, or at least not as much as it was about the threat of Eta terrorism," he wrote.
"If that is the case, the government undoubtedly has to bear a responsibility.
"Perhaps the very successes achieved in the
fight against Eta in recent years led us to lower our guard
against the fundamentalist threat."
Mr Aznar's premiership was marked by a campaign against the Basque militant separatist movement, which led to the arrest last year of more than 150 suspected Eta members by Spanish and French police.