The Syrian Democratic Forces claimed she was killed by members of the Islamist militant organization Ahrar al-Sharqiya, an anti-Assad rebel group now fighting for Turkey.
While questions swirled around her death, videos were posted by the Turkish-backed Syrian rebel group Ahrar al-Sharqiya on social media that appeared to be filmed by her killers, and which showed them mock her and her companions, as described by The Associated Press. The videos apparently show unarmed civilians with their hands tied as they're shot.
ABC News confirmed that the politician, a human rights advocate, was shot in the face, but reports that she had been raped were untrue.
"Turkish-backed terrorist groups are committing war crimes in NE Syria," British politician Lloyd Russell-Moyle tweeted on Sunday, the day after the killings. "Filmed sectarian roadside executions recall IS tactics. Future Syria party leader Hevrin Khalaf has been executed. Her killers filmed it on their phones."
Russell-Moyle's tweet echoed calls from around the world labeling these killings -- and other actions taken by the Turkish military and Turkish-backed fighters -- as war crimes. But actually labeling, let alone getting justice for, an action viewed by many as a "war crime" is more complicated, especially given the region's increasingly complicated politics.
As civil war in Syria continued through the early part of this decade, between attempts to overthrow Assad and the rise of ISIS, the U.S. entered the scene and worked with local forces, including the Kurdish People's Protection Units, a local militia that worked to protect Kurdish areas from attack. But Trump entered office with the intent of pulling troops out of the country, and in October, seven months after declaring the end of ISIS, he announced troops would be withdrawing.
This announcement came after a call with Turkey's President Recep Tayyip Erdogan. Within a week of that call, Erdogan announced Turkey had begun an operation in Syria, including targeting the Kurdish forces the U.S. had supported, and Khalaf was dead.
A State Department spokesperson confirmed seeing "reports of the killing of" Khalaf and found them "extremely troubling," adding that the U.S. will "condemn in the strongest of terms any mistreatment and extrajudicial execution of civilians or prisoners."
"President Erdogan bears full responsibility for its consequences, to include a potential ISIS resurgence, possible war crimes and a growing humanitarian crisis," U.S. Secretary of Defense Dr. Mark T. Esper said in a statement Monday about Turkey's overall actions in Syria.
The Geneva Conventions of 1949, which included Turkey, clearly state that "murder of all kinds," "torture," "humiliating and degrading treatment" and "the carrying out of executions without previous judgment pronounced by a regularly constituted court" of people "taking no active part in the hostilities" are war crimes.
"If a politician is driving away from the fighting, not participating at all, her killing would be a murder, and murder is clearly prohibited in the Geneva Conventions," Mary Ellen O'Connell, a professor of international law at Notre Dame, told ABC News.
A spokesman for the United Nations human rights office said U.N. war crimes investigators would be following up on incidents in Syria, Reuters reported Tuesday.
But "the question of accountability is a complicated one," Sarah Cleveland, faculty co-director of the Human Rights Institute at Columbia Law School, told ABC News.
There are various people who could be held accountable for a war crime -- the individuals who committed the actions, their commanders and up through the ranks depending on who's deemed responsible. Turkish officials could be held responsible for the actions of Syrian fighters if it's proven they directed the fighters.
Then, there are several avenues through which war crimes and human rights violations can be prosecuted.
Probably the most well-known avenue is that the U.N. Security Council can refer cases to the International Criminal Court in The Hague. However, Cleveland cautioned, given that Russia is a permanent member of the Security Council with veto power -- and since Russia is playing its own complex role in Syria and in relation to Turkey -- it's less likely that the ICC will see a case.
Another option is that countries not involved in a certain conflict have the ability to exercise jurisdiction over war crimes. Germany, for example, has arrested former Syrian officials for crimes against humanity and is holding trials.
A third possibility, floated by some in the international community, is to set up a tribunal, which can be established within the United Nations to address a broad range of crimes in a particular region. Tribunals were set up for the former Yugoslavia to address war crimes in the Balkans in the '90s and for Rwanda to address the genocide there in the '90s, resulting in indictments and imprisonment for dozens.
So far, setting up a tribunal has primarily been discussed in the context of prosecuting ISIS militants, but one could be broadened to also address other potential war crimes in Syria, including actions committed by Assad and his forces and by Turkey and fighters supported by that country.
Additionally, the European Court of Human Rights could take up a human rights-based case should individuals or states submit one.
As O'Connell, the Notre Dame professor, sees it, focusing on individual murders by Turkish or Turkish-supported fighters misses a bigger point: "Turkey's cross-border operations in Syria are a violation of the U.N. Charter prohibition on the use of force. It is an act of aggression."
"Aggression is the most serious war crime you can commit," she continued. "It means that all of the killing, all of the destruction that follows from that unlawful decision to use military force is unjustified."
She doubts that will be prosecuted, however, as nations like Russia and the U.S. are "not coming to this argument with clean hands" given their military actions in Crimea and in Iraq in 2003, respectively.
Rather than turn to prosecution, O'Connell believes the international community should rally together to use this moment as "a chance for a reset, to get back to the law that everyone was committed to before 9/11." The international community, she said, should urge Turkey to pull back and be part of regional negotiation to bring an end to the Syrian civil war and ISIS.
This is something Vice President Mike Pence could have the opportunity to do as he leads a delegation to Turkey this week to attempt to negotiate a ceasefire and settlement between Turkey and U.S.-backed Syrian Kurdish forces.
"If that message isn't brought home to Turkey, the worst-case scenario is extraordinarily grim," O'Connell said.
But for Cleveland, of Columbia Law School, focusing only on Turkey's actions has the possibility of missing an even broader range of potential war crimes and human rights violations committed in the region over the last decade.
"Basically every form of violation of international, humanitarian law possible has been committed in Syria, from the use of chemical weapons to intentional targeting of civilians and hospitals and schools, to indiscriminate bombing of heavily occupied civilian areas," she said. "The list goes on and on."