Who carries out investigation for election-related criminal offenses?

Latvia
Answer
Public prosecutor
Source
Criminal ProcedureLaw (as of 2010, in English)
Source: http://www.knab.gov.lv/uploads/eng/criminal_procedure_law.pdf
Section36.(1) A public prosecutor in criminal proceedings shall realise investigationsupervision, investigation, criminal prosecution, the maintenance of Stateprosecution and other functions specified in this Law.