More than 250 people including 42 foreigners were killed in the attacks, for which Islamic State has claimed responsibility.

The bombings sent shock waves through the Indian Ocean island state, which had enjoyed relative peace since a civil war ended a decade ago.

"You are hereby directed to initiate criminal investigations with regard to the lapses of the above officers to prevent/minimise the above attacks," the Attorney General wrote in a letter to the acting police chief seen by Reuters.

The letter named the nine senior police officers.

Despite three advance intelligence reports from India on pending attacks, Sri Lanka's top defence officials failed to act before the Easter day suicide bombings by Islamist militants that targeted three churches and three luxury hotels.

Authorities said the attacks were carried out by two little-known domestic groups - the National Thawheedh Jamaath (NTJ) and Jamathei Millathu Ibrahim.

Authorities say the threat of more attacks has been contained and the security services have dismantled most of the network linked to the bombings.

Australian Associated Press