Sirisena requests resignations of defence secretary, inspector general of police over failure to act on threat warnings.
A series of coordinated bombings on Easter Sunday rocked Sri Lanka killing more than 350 people.
At least 500 people have been wounded in the deadliest attack in the island nation since the end of the civil war 10 years ago.
The blasts targeted three churches, as well as four hotels – including the Shangri-La, Kingsbury and the Cinnamon Grand – in the capital Colombo.
Nearly all victims were Sri Lankan. Dozens of foreigners were also killed.
Sri Lanka’s state defence minister said initial investigations showed the attacks were carried out by two little-known Muslim organisations.
Authorities have asked media organisations reporting on developments related to Sunday’s attacks to use material confirmed by official spokespeople for police, the armed forces, or the Department of Government Information.
A notice issued by the department said caution was necessary to prevent the spread of false information.
The guidelines came as the Sri Lankan government maintained a social media ban, saying it would stop the spread of false reports that might incite violence.
All Catholic churches in the country have been instructed to stay closed and suspend services until security improves. There will be no religious gatherings at Catholic churches on Thursday.
“On the advice of the security forces we are keeping all churches closed,” a priest told AFP.
Security has been beefed up for Cardinal Malcolm Ranjith and for the Bishop’s House in Colombo, sources have told Al Jazeera.
Sri Lanka’s government said Thursday it was suspending plans to grant citizens of 39 countries visa-free entry during the country’s tourism low season in the wake of the deadly Easter bombings.
“Investigations have revealed foreign links to the (Easter) attacks and we don’t want this programme to be abused,” Tourism Minister John Amaratunga said in a statement.