The Committee to Protect Journalists (CPJ) want the Tanzanian authorities to carry our credible investigations and publicly account for a freelance journalist Azory Gwanda who disappeared one and half years ago.
Gwanda was last seen by his family and friends on November 21, 2017, according to the CPJ.
His wife Anna Pinoni said he told her he was going on an emergency trip and would return the following day but he never came back.
Ms Pinoni told the Mwananchi newspaper that she thought her husband's disappearance might be linked to his articles on a series of mysterious killings in Tanzania's coastal region, a view shared by others with whom CPJ has interviewed.
Friday, April 5, will mark the 500th day since his disappearance.
CPJ on Thursday launched the #WhereIsAzory campaign to bring attention to the case.
"Azory Gwanda is a freelance journalist reporting about his community and he must not become just another statistic," said Ms Angela Quintal, CPJ's Africa program coordinator.
"Through this campaign, we want to ensure that Gwanda's case becomes a priority for the Tanzanian authorities and that we get much-needed answers about what really happened him. Until that time, Tanzanian journalists will not feel safe."
CPJ is urging Tanzanians to participate by sharing the hashtags #WhereIsAzory and #MrudisheniAzory on social media.
Jounalists in Tanzania and other media outlets are wary of retaliation, CPJ says, if they are too vocal about Gwanda's case.
His disappearance comes amid declining press freedom in the country, including government-ordered media shutdowns, fines, restrictive regulations, and arbitrary arrests of journalists.
CPJ Africa will highlight Gwanda's case on Twitter, Facebook, and Instagram.
Last year, two CPJ representatives, Ms Quintal and Muthoki Mumo, were detained overnight and interrogated in Tanzania last year, they were specifically asked about their interest in Gwanda.
The incident happened in Dar es Salaam when state operatives at the Southern Sun Hotel.