Sri Lankan police have arrested three people and seized 21 locally-made grenades and six swords during a raid in Colombo, a police spokesman says.
He did not give further details or suggest that the raid was linked to suicide bomber attacks on Easter Sunday that killed 359 people and wounded 500.
More people, including foreigners, were swept up for questioning overnight as authorities probed deeper into the bombings, which were potentially the deadliest operation claimed by Islamic State.
Police said an Egyptian and several Pakistanis were among those detained, although there was no immediate suggestion they had direct links to the attacks.
Police on Thursday detained 16 additional people for questioning overnight, taking the number detained since Sunday to at least 76.
Earlier on Thursday, Sri Lankan authorities scrambled to lock down the central bank and briefly shut the road leading to Colombo airport.
Two bank officials told Reuters the street outside the building near the World Trade Centre in the capital was blocked before the security alert was lifted.
Underscoring the nervous atmosphere in the Indian Ocean nation, authorities also shut the entry road to Colombo's main airport after a suspicious vehicle was identified at a nearby car park, which proved a false alarm.
A police spokesman also said there was an unexplained explosion in a town east of the capital, but there were no casualties.
Of those who carried out the attacks, a picture has slowly emerged of a group of nine well-educated, home-grown Islamist suicide bombers, including a woman.
However, Sri Lankan and international authorities have also focused investigations on international links to domestic Islamist groups National Thawheed Jama'ut and Jammiyathul Millathu Ibrahim.
Islamic State claimed responsibility for the attacks but offered no firm information to back up its claim.
The Islamist group released a video on Tuesday that showed eight men, all but one with their faces covered, standing under an Islamic State flag and declaring their loyalty to leader Abu Bakr Al-Baghdadi.
The one man in the video with his face uncovered has been identified as Mohamed Zahran, a preacher from the east of Sri Lanka known for his militant views who may have been the attack's mastermind.
Australian Prime Minister Scott Morrison said another of the bombers had lived in Australia with his wife and child on a student visa but left the country in 2013.
Morrison did not identify the man, although his family have said his name was Abdul Latheef Mohamed Jameel.
The bombings shattered the relative calm that has existed in Buddhist-majority Sri Lanka since a civil war against mostly Hindu, ethnic Tamil separatists ended 10 years ago, and raised fears of a return to sectarian violence.
President Maithripala Sirisena will meet representatives of different faiths later on Thursday to address concerns of a sectarian backlash.
