"The circumstances of this tragedy must be swiftly and thoroughly investigated," Mijatovic said in a statement on Monday.
Mijatovic said that the journalist's death was a loss for media community.
Read alsoFBI experts analyze composition of explosives planted under Sheremet's carShe also urged the Ukrainian authorities to "improve the fragile situation regarding the safety of journalists and fully and effectively address the issue of impunity."
Mijatovic expressed sincere condolences to Shchetinin's family, friends and colleagues.
On August 28, Shchetinin, a journalist with and founder of the Novy Region online news agency, was found dead in his apartment in Kyiv with a gunshot wound.