The bodies of 60-year-old Orouba Barakat and her 22-year-old daughter Halla were found overnight in their apartment in the Uskudar neighborhood on the Asian side of the city.
According to reports in Turkish media, the women were strangled and stabbed to death four or five days ago. Police officials told ABC News that an investigation is underway but no arrests have yet been made.
The elder Barakat was an activist with the Syrian Opposition Council, a group of Syrian expatriates opposing President Bashar Assad and his government. Halla Barakat was working as an editor for the pro-opposition website Orient News and freelancing for other media outlets, including ABC News.
An unidentified journalist who knew Halla Barakat told the Committee to Protect Journalists that both women had received death threats via social media and email.
The US State Department has released a statement saying it was "deeply saddened" by their deaths.
"The United States condemns the perpetrators of these murders and we will closely follow the investigation," the statement read.