A Syrian activist and journalist known for his work exposing the brutality of the Islamic State group (ISIS), was shot dead in broad daylight on Sunday in the southern Turkish city of Gaziantep.

Naji Jerf, 37, was walking down the street when a gunman shot him in the head, according to local reports. No one has claimed responsibility for the killing yet, but some ISIS supporters celebrated Jerf's death with posts on social media. Some local news outlets believe Jerf's well-known reporting on ISIS may have been the reason he was targeted.

See also: Meet the activists risking their lives to tell the truth about ISIS

In mid-December, Jerf posted a documentary on the rise of ISIS in Aleppo, which had aired on Arab-language news outlets. The film exposed the murder of Syrian activists by ISIS during the group's occupation of Aleppo in 2013 and 2014. It aired on Al-Arabiya and was viewed more than 12 million times on the outlet's YouTube channel.

Jerf was also the editor-in-chief of an independent monthly magazine called Hentah, which supports the Syrian opposition. A day before he was killed, Jerf posted a photo to his Facebook page showing him reading the latest edition of the magazine.

Photo posted by Naji Jerf to his Facebook page on Dec. 26, 2015.

The Committee to Protect Journalists (CPJ) condemned his murder and called on Turkish authorities to protect other journalists in the country.

"Syrian journalists who have fled to Turkey for their safety are not safe at all," said Sherif Mansour, CPJ's Middle East and North Africa program coordinator in a statement released Sunday. "We call on Turkish authorities to bring the killers of Naji Jerf to justice swiftly and transparently, and to step up measures to protect all Syrian journalists on Turkish soil."

Jerf recently created a documentary on the group Raqqa is Being Slaughtered Silently, which has also been a target of ISIS for their reporting. Raqqa is Being Slaughtered Silently uses social media to distribute photos and videos of from inside ISIS controlled territory in Syria. The group posted about Jerf's death on their Twitter account.

Our movie director Naji Jerf "father of 2 kids" was assassinated by suppressor gun today in Gaziantep "#Turkey" pic.twitter.com/F3TFZyAwk9 — الرقة تذبح بصمت (@Raqqa_SL) December 27, 2015

The journalists who ISIS is targeting

This is not the first time anti-ISIS activists and citizen journalists have been targeted, both in territory they control inside Iraq and Syria, and across the border in Turkey. In the spring of 2014, ISIS captured one of the founding members of Raqqa is Being Slaughtered Silently, Moataz Billah, and publicly executed him in Raqqa.

Using Billah's phone, the militants mass messaged the citizen journalist group via Facebook and threatened to hunt them down and kill them.

The father of another member of the group was killed in Raqqa in July 2015. In October, two citizen journalists, one of them a member of Raqqa is Being Slaughtered Silently, were beheaded in the Turkish city of Urfa, near the Syrian border.

“We get death threats daily," Abdalaziz al-Hamza, a spokesperson for the group told Mashable in November.

"All of us have received death threats from ISIS but we thought at least some of us should show our faces, to show the world we genuinely are from Raqqa," said Hamza.

Hamza, who is based in Berlin, posted a photo to his Facebook page on Monday showing Jerf's coffin with a photo on top.

Jerf was the father of two young daughters and according to the BBC, he has recently been granted asylum in France and was due to travel to Paris this week.

He was originally from the Syrian city of Salamiyah in western Syria, according to his Facebook page. While living in Syria, Jerf covered human rights abuses by the Assad regime and coordinated the work of local citizen-journalists after the start of the country's uprising in 2011, according to Reporters Without Borders. Government authorities ransacked his office in October 2012, and he soon fled to Turkey.

He was buried on Monday. Footage posted by a local journalist from his burial shows Jerf's wife sobbing over her husband's coffin.

Wife of slain #Syria activist and filmmaker Naji al-Jerf, gunned down Sunday in Turkey, says her final goodbyes. pic.twitter.com/4DJJrFHABN — Borzou Daragahi (@borzou) December 28, 2015

In the wake of Jerf's death, thousands have shared his most recent documentary about ISIS in Aleppo on social media.