A poster of Syrian President Bashar al-Assad is displayed outside Syria’s embassy in Bucharest, Romania on March 17. (Vadim Ghirda/Associated Press)

The Syrian defector known as “Caesar,” who brought the world the largest trove of evidence of mass atrocities perpetrated by the regime of Bashar al-Assad, is returning to Washington this weekend. Three years after he helped expose some of the worst war crimes of our generation, the victims of those crimes are still a long way from getting justice.

From 2011 to 2013, Caesar worked as a military photographer in the Syrian army, forced to meticulously document the torture and murder of thousands of men, women and children inside Assad’s jails. When he fled Syria in 2013, he brought with him over 55,000 images that show the killing of over 11,000 civilians in custody, along with documents detailing the Syrian government’s highly organized system of mass murder.

The photos, some of which were released publicly in 2014, show bodies starved, tortured and mutilated. The Syrian government kept detailed records. Assad’s “machinery of death” was the worst since the Nazis, the State Department’s ambassador-at-large for war crimes Stephen Rapp said at the time.

Caesar testified before Congress in the summer of 2014 and explained to U.S. lawmakers that the evidence he smuggled out of Syria showed only a small segment of the overall government operation and that tens of thousands of civilians were still being tortured and murdered in Assad’s prisons.

The U.S. Holocaust Memorial Museum took up Caesar’s cause and helped build an effort to raise public awareness about the Syrian government’s mass atrocities, working with elements of the Syrian opposition. Caesar’s photos were shown in the halls of Congress, the United Nations and the European Parliament. In 2015, the FBI verified the authenticity of the photos after an extensive forensic investigation.

Now, Caesar is returning to the United States with a simple question: What progress has been made? For those pushing for accountability, justice and a halt to the slaughter, the sad answer is not enough.

“I think he’s coming to Washington because he wants answers about why more hasn’t been done and why his evidence hasn’t done more to galvanize support for helping the Syrian people,” said Cameron Hudson, the director of the Holocaust Museum’s Center for the Prevention of Genocide. “Those are the same answers we’ve been searching for and I don’t think he’s going to get them.”

Caesar’s second visit to Washington will be decidedly lower profile than his first. Next week, the Holocaust Museum will host a small, private event featuring him and a more public event on Capitol Hill about the Caesar file that Caesar himself will not attend. Originally, Caesar had been invited to testify at the Senate Foreign Relations Committee hearing on March 15, the sixth anniversary of the start of the Syrian crisis. But due to travel difficulties, he couldn’t reach Washington in time.

In fact, if the Trump administration’s new executive order restricting immigration from six mostly Muslim countries had gone into effect March 16, as was anticipated, Caesar might not have been able to get his visa at all.

The Holocaust Museum’s event on Capitol Hill will include participation by several congressional leaders. “After six years, the horrific human toll of the conflict continues to be felt throughout the region, across Europe and beyond,” Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.) told me.

The Trump administration is still developing its Syria policy and has said very little about its plans, other than promising to consider safe zones and vigorously go after the Islamic State. There are some inside the White House who advocate cooperating closely with Russia in Syria, despite Russia’s complicity in Assad’s war crimes. A similar debate played out inside the Obama administration.

“It is the preeminent mass atrocity event of this generation and of the Obama presidency. And we will have to see if this is the preeminent mass atrocity event of the Trump administration as well,” Hudson said.

Obama’s White House shied away from confronting Assad on war crimes because top officials believed that might complicate efforts to find a negotiated political solution to the crisis. Assad would never leave power if he thought it would result in his arrest and trial before an international court, the argument went.

For Trump, the calculation is completely different. Assad is under less pressure to leave power than he has been since the war began. The Trump administration has a stated policy of not interfering in the internal affairs of other countries. President Trump has repeatedly said that Assad is better than the alternative.

The problem is, the effects of Assad’s war crimes against his own people are not contained inside Syria, Rapp told me. Now retired from government service, Rapp has been intimately involved in the effort to document, verify and then use various repositories of evidence of war crimes and crimes against humanity to hold the Assad regime accountable. Assad’s atrocities fuel extremism, drive civilians to become refugees and encourage other bad actors to feel safe to do the same or worse, he said.

“There are certain crimes that are so horrible that they are not internal affairs. When you commit mass torture and mass killings, those are not internal affairs. They concern the whole world,” Rapp said. “This is not an internal affair, just as terrorism is not an internal affair when it happens in one country.”

Action by the International Criminal Court has been blocked for years by Russia, which can veto referrals at the U.N. Security Council. But the U.N. General Assembly did pass a resolution last December establishing a mechanism to support the investigation and documentation of serious war crimes in Syria. Also, several victims identified in the Caesar file have European nationalities, giving each of those countries jurisdiction to prosecute the cases.

In Congress, there’s a bill named after Caesar that would increase sanctions on the Assad regime and its Russian and Iranian partners for their involvement in war crimes in Syria. The House passed the bill last year but the Senate failed to act. The sanctions could give the Trump administration leverage to use to halt some of the killings.

For Caesar, his hope is that his efforts and his sacrifices won’t go to waste. He now lives in exile in a European country with no means of making a living and in constant fear the Assad regime will exact retribution against him, his family or his friends. The very least Washington can do is let him know that the United States will do everything it can to make sure his evidence does matter, said Rapp.

“This administration needs to support this effort,” said Rapp. “These are enormous crimes. It’s never too late for justice as long as there are surviving witnesses and people to prosecute. That’s why Caesar’s visit is important now.”