An explosion went off on Monday in a van near a church in Colombo where scores were killed the previous day.
It happened when bomb squad officials were trying to diffuse the explosives left inside the vehicle, which police say was used by the attackers on Sunday.
It was found close to Kochchikade Church where scores of people were killed.
The death toll from bomb blasts that ripped through churches and luxury hotels in Sri Lanka rose dramatically on Monday to 290 - including dozens of foreigners.
Officials say 24 people have now been arrested in connection with the attacks.
Sri Lankan authorities believe a previously little-known local group called National Thowheed Jamath carried out the attacks.

US, Iran inch closer to deal, timing remains unclear
Sirens sound in Jordan due to technical malfunction, state TV reports
India Air Force plane crash kills all five onboard
Ukrainian strike kills one, wounds 3 in southern Russia
Trump says US military strike killed leader of Venezuela's Tren de Aragua gang
Qatar rejects media report on energy production decisions
Trump says Iran war deal close as Strait of Hormuz tensions linger
Victims' families mark Air India crash anniversary with prayers and tributes
