COLOMBO:  Sri Lanka's attorney general advised the acting police chief on Monday to launch a criminal investigation of the former defence secretary over "major lapses" that contributed to security failures ahead of Easter suicide bombings that killed more than 250 people.

Attorney General Dappula de Livera said he decided to recommend an investigation of Hemasiri Fernando based on the findings of a special board of inquiry appointed by President Maithripala Sirisena after the April 21 blasts.

Fernando stepped down four days after the blasts after Sirisena asked for his resignation and that of police chief Pujith Jayasundara, who refused to resign. Sirisena later suspended Jayasundara and appointed acting police chief.

Sirisena appointed the board of inquiry amid criticism that he could have prevented the attack. After investigating for several weeks, the board submitted its report to Sirisena, who sent it to de Livera for his consideration.

"You are hereby advised to take steps to initiate criminal investigations with regard to the major lapses attributed to former Secretary, Ministry of Defense, Mr Hemasiri Fernando, for his failure to prevent/minimise the above attacks," de Livera said in a letter sent to acting police chief Chandana Wickremeratne on Monday.

In the letter, seen by The Associated Press, de Livera said he considered the contents of the final report of the special board.

Fernando has not commented on the accusations.

Last week, de Livera also advised the acting police chief to initiate a criminal investigation into nine senior police officers for "lapses" that led to the failure to prevent and minimise the attack. The nine officers served in the areas where the attacks took place.

More than 250 people were killed when seven Sri Lankans who had pledged allegiance to the Islamic State group blew themselves up at three churches and three luxury hotels. Some 500 people were wounded.

Sri Lankan leaders and the security establishment are under fire for not acting on near-specific intelligence information on possible attacks on churches. Government leaders have acknowledged that some intelligence units were aware of possible attacks weeks before the bombings.

Sirisena said he had been kept in the dark on intelligence about the planned attacks and vowed to "take stern action" against officials who failed to share it.

A parliamentary committee is also looking into intelligence failures, despite objections by Sirisena after some officials hinted at shortcomings by the president, who is also the defence minister and minister of police.