The suspect was arrested in Mahawa.
The suspect was found to be in possession of 18 rounds of live ammunition.
The hand grenade was found in a toilet on the first floor of the hospital earlier this week.
Colombo Crimes Division had arrested a 26-year-old suspect from Uppuveli, Trincomalee over the incident previously.
The suspect had placed the hand grenade and had given the tip off himself, with the objective of claiming a cash reward for this act.
Police said the safety pin of the hand grenade had been removed and the grenade was attached to a mosquito coil.
The Colombo Crimes Division is conducting investigations.
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