Zahran Hashim, who blew himself up at a hotel in Colombo founded the group National Thowheed Jama’ath (NTJ), which has now been proscribed
Police have raided NTJ’s headquarters in the eastern town of Kattankudy.
The Sri Lankan president also announced a ban on the niqab and the burqa, targeting Muslim women following the terror attacks.
The bombings targeted churches and hotels, killing 253 people.
Sunday church services were cancelled across Sri Lanka as a precaution but worshippers in Colombo gathered to pray outside St Anthony’s.
Security forces raided a house in Sainthamaruthu, near Hashim’s hometown Kattankudy, on Friday 26 April.
Gunmen opened fire as police officers and soldiers closed in, and three men set off explosives, killing themselves, six children and three women, police say. Three other people died in gunfire.
Police sources who spoke to Reuters news agency named the three men as Mohamed Hashim, and his sons Zainee Hashim and Rilwan Hashim.
Reuters added that all three men were filmed on a social media video circulating calling for all-out war against all non-Muslims.
In Kattankudy itself, police searched the HQ of NTJ which Zahran Hashim had led.