(CNN) Mourners gathering to commemorate the 40th day since the death of Nika Shahkarami, an Iranian teenager who went missing on September 20 after attending protests in the capital Tehran, were met with gunfire and tear gas by security forces, according to video posted on social media and geolocated by CNN.
Video showed apparent smoke from tear gas near a bridge where mourners had gathered near the village of Veysian. Apparent gunfire was audible in another video clip shared on social media. It is unclear if there were any casualties.
The teen's maternal aunt, Atash Shahkarami, had posted a photo on Instagram saying that the 40th-day commemoration would be held on Thursday at a cemetery in Lorestan province.
Video shows mourners holding photos of Shahkarami and chanting "Khamenei will be overthrown this bloody year," "We are all Nika, come on fight," "Death to the dictator," and "Death to Khamenei."
When the 16-year-old went missing, she was attending one of the protests that sprung up throughout the country against the death of Mahsa Amini, a 22-year-old Kurdish-Iranian woman who died last month in state custody after being detained by the country's morality police.
On Thursday, CNN published an exclusive investigation into the final hours of Shahkarami's life.
Iranian authorities claimed Shahkarami's body was found at the back of a courtyard on the morning of September 21. Her mother wasn't given access to identify her until eight days later.
CCTV footage released by the authorities timestamped just after midnight as September 20 became September 21 showed the figure of a masked person they said was Shahkarami entering a building that was uninhabited, and still under construction in Tehran.
A Tehran prosecutor initially said she died after being thrown from the building's roof, and that her death "had no connection to the protests" of that day, but despite apparently declaring her death a homicide, he did not say whether there were suspects under investigation.
But dozens of videos and eyewitness accounts obtained exclusively by CNN indicate that Shahkarami appears to have been chased and detained by Iranian security forces that night. One key eyewitness, Ladan, told CNN she saw Shahkarami being taken into custody at the protest by "several large-bodied plainclothes security officers" who bundled her into a car.
On Wednesday, after CNN asked the government to comment on the evidence in this investigation, an Iranian media report quoted a Tehran prosecutor as saying that Shahkarami's death was a suicide. Iranian authorities still have not responded to CNN's repeated inquiries about her death.
The Thursday clampdown of mourners in Veysian comes after clashes broke out throughout Iran as people tried to mark 40 days since Amini died, an important day of mourning in Iranian and Islamic tradition.
On the same day, a Shia holy shrine in the city of Shiraz was attacked on Wednesday that left at least 15 people dead and more than 40 injured, ISIS affiliated Amaq news agency said. ISIS has claimed responsibility for the attack, but Iran's leaders have conflated the attack with anti-government protesters.
On Thursday, Iran's Supreme Leader Ayatollah Seyyed Ali Khamenei vowed to take action against the attackers. "The perpetrator or perpetrators of this lugubrious crime will certainly be punished," he said.
Iranian military commanders also threatened anti-government protesters with further crackdowns, after claiming they were "complicit" in Wednesday's attack, according to statements published by the official state news agency IRNA.
The commanders did not provide evidence for the claim.
Iran's Armed Forces said in a statement that the protests were designed to create "insecurity and chaos" and added "this terrorist act is also a part of that design."
Foreign Minister Hossein Amir-Abdollahian said that evidence from the attack showed that "foreign meddlers" have "designed a multi-layer project" to weaken the country.
"We will definitely not allow Iran's national security and interests to be toyed with by terrorists and foreign meddlers who claim to defend human rights," Foreign Minister Hossein Amir Abdollahian said in a statement carried by the foreign ministry.