The two staffers were detained in Dar es Salaam on Wednesday evening.
Quintal is the Africa programme coordinator at the Committee to Protect Journalists and Mumo is the Sub-Saharan Africa representative for the press freedom NGO.
They were detained on Wednesday by "officers who identified themselves as working with the Tanzanian immigration authority", according to the organization.
"Angela Quintal and Muthoki Mumo traveled to Tanzania to understand the challenges facing the Tanzanian press and to inform the global public", Committee to Protect Journalists executive director Joel Simon said in a statement.
Quintal tweeted before 1am on Tuesday morning that she and her colleague had been released, but Quintal's family raised concerns that the tweet did not sound as if it had come from Quintal whose Twitter account was subsequently suspended.
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Earlier on, Quintal's sister Genevieve Quintal faulted information shared through her Twitter account, saying that it was shared by Tanzanian police. They were released on Thursday morning but are yet to get their passports back. "This shows someone is using her account".
The duo were arrested in their hotel room in Dar es Salaam and had their passports confiscated.
"Sanef calls on the Tanzanian authorities to return their passports as a matter of urgency and allow them to leave the country of their own free will".
Other freedoms are also under pressure, with a powerful Dar-es-Salaam official last week announcing an anti-gay witch-hunt to track down people suspected of engaging in homosexuality, which is illegal in Tanzania under British colonial-era laws.
The new regulation gives the government the right to revoke the permit if a website publishes content "that leads to public disorder" and "threatens national security".