Paulo Machava was shot by unknown assailants on Friday, while on his morning jog in Maputo.
"We don't know the motives for the killing but we will work hard to trace the killers," police spokesman Orlando Modumane told The Associated Press on Saturday.
A private security guard found Machava's body lying in a pool of blood. He said he heard four gunshots before seeing young men drive away.
"They're professionals," the guard told journalists, saying they moved quickly, using AK-47 assault rifles. Machava's brother Januario told reporters he believed the shooters trailed their victim, studying his daily routine.
Also Read
He was the founder and editor of a news website at the time of his death.
Machava's murder created "a climate of fear, compromising press freedom," the Media Institute of Southern Africa said in a statement.
"This is once more a way of trying to silence journalists in our country," said Eduardo Constantino, the chair of the Mozambique's National Union of Journalists.