Who carries out investigation for election-related criminal offenses?

Presidential Elections Act(No. 15 of 1981) section 68.

….

(3) Any police officer maytake such steps and use such force as may be reasonably necessary forpreventing any contravention of the provisions of subsection (2) and may seizeany apparatus used for such contravention.

source: http://www.commonlii.org/lk/legis/num_act/pea15o1981280/s68.html