Experts warned Wednesday of “mounting evidence” that genocide is being committed against Myanmar’s stateless Rohingya minority, citing new testimony from witnesses to recent military violence that has sent more than half a million people pouring over the border into neighboring Bangladesh.

The U.S. Holocaust Memorial Museum and Fortify Rights, an NGO that investigates human rights violations in Southeast Asia, said in a report that Myanmar’s state security forces appear to have “targeted the Rohingya group with several of the enumerated acts in the law of genocide.” The U.N. has previously said recent violence in the country amounts to ethnic cleansing.

“The Government of Myanmar not only ignored warning signs… but also created and perpetuated an environment for mass violence and atrocities,” reads the report, which was based on more than 200 in-depth interviews and other research carried out over the past year.

The warning comes as Secretary of State Rex Tillerson arrived in Myanmar where he will meet with the country’s de facto leader, Nobel Peace Prize laureate Aung San Suu Kyi, who has come under criticism for failing to stop or condemn or the violence. State Department officials told The Washington Post that Tillerson will press for a credible investigation of alleged abuses and could raise the threat of targeted sanctions.

Read more: Myanmar’s Attempt to Destroy Rohingya Muslims

Beginning with an initial wave of violence in Oct. 2016, soldiers and civilian mobs have been accused of brutal rampages through Rohingya villages, shooting, stabbing and raping before razing homes and property. A second, larger campaign began in late August and triggered a massive exodus, sending more than 615,000 Rohingya over the border to Bangladesh – more than half the entire Rohingya population. Most of those who fled were children.

The Myanmar government has denied the atrocities, and the military this week attempted to exonerate itself through an internal investigation dismissed as a farce.

Both waves of the military-led operations followed fatal attacks by Rohingya militants on security outposts. But the report says these attacks were used as a pretext to carry out a campaign of ethnic cleansing. Military-led violence against Rohingya men, women and girls has since created what the U.N. called the world’s fastest-growing refugee crisis as Bangladesh struggles to provide aid to the sudden influx of traumatized, desperate and severely malnourished refugees.

“The Rohingya have suffered attacks and systematic violations for decades, and the international community must not fail them now when their very existence in Myanmar is threatened,” Cameron Hudson, director of the Simon-Skjodt Center for the Prevention of Genocide at the U.S. Holocaust Memorial Museum, said in a statement.

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Write to Laignee Barron at Laignee.Barron@time.com.