Mohib Ullah, a Rohingya community leader who believed in the power of data to confront the brutality of ethnic cleansing, died on Wednesday, shot by gunmen in a bamboo and tarp shelter in Kutupalong, Bangladesh, the world’s biggest refugee camp. He was 46.

The gunmen had burst into his shack before opening fire, according to his brother, Habib Ullah, who was with Mr. Mohib Ullah at the time. The shack was stacked high with papers documenting massacres of Rohingya, the Muslim minority native to neighboring Myanmar.

Mr. Mohib Ullah, who had worked as an administrator and teacher in Myanmar, quickly emerged as a leader in the sprawling refugee settlements of southeastern Bangladesh, which houses about a million displaced Rohingya, many traumatized by their escapes from a campaign of killing, rape and village burning by the Myanmar military.

His clerklike appearance — crisply parted hair, clean shaven, pens in his shirt pocket — belied a determination that earned him respect in a community that usually venerates clerics and men with gray in their beards. Yet his willingness to speak unpalatable truths also earned him enemies, even within the Rohingya camps. Death threats became part of his life.

Mr. Mohib Ullah escaped Myanmar in 2017, when his village, like hundreds of others, was torched by the Myanmar military in a violent campaign that United Nations investigators said bore the hallmarks of genocide. He had barely settled in his tarp shelter before he began trying to document the Myanmar soldiers’ crimes. For years he painstakingly knocked on the doors of refugees, compiling a list of the dead, checking and cross-referencing each life lost. The aim was to provide evidence for international courts to one day prosecute the Myanmar military for genocide and war crimes.

When the Rohingya wanted to mark the anniversary of the August 2017 massacres that catalyzed their largest exodus into Bangladesh, Mr. Mohib Ullah tackled the logistics of organizing rallies that took place against the wishes of Bangladeshi security forces. He spoke with gentle force.

With his English skills and quick smile, he also became an ally of Bangladeshi and international nongovernmental organization workers, who were trying to manage the influx of Rohingya into the country. He started an N.G.O. called the Arakan Rohingya Society for Peace and Human Rights, using another name for Rakhine State, the home of the Rohingya in western Myanmar.

“He was a little big man,” said Yanghee Lee, the former United Nations special rapporteur for Myanmar, who last saw Mr. Mohib Ullah in the camps last year and spoke with him by phone last month. “He was all smiles, committed and firm and fearless and courageous.”

“I don’t have enough adjectives to explain how dedicated he was to his fellow Rohingya brothers and sisters,” Ms. Lee added.

Mr. Mohib Ullah traveled to Europe and the United States to raise awareness of the plight of Rohingya Muslims, who have endured decades of state persecution in Myanmar. Many had their citizenship essentially stripped from them after a xenophobic military dictatorship targeted ethnic minorities. By the 2000s, once-vibrant Rohingya communities were depleted, as the authorities limited their worship, education and health care. The Myanmar authorities mandated that Rohingya women control the number of children they bore so that the Muslim population of Rakhine State would not compete with the Buddhist one.

After a civilian government began sharing power with the military in 2015, the pogroms against the Rohingya intensified. Elected leaders and military officers alike maintained that no such group called the Rohingya existed, referring to them instead as Bengalis, to imply that they were interlopers from Bangladesh rather than an ethnic group that called Myanmar home.

In a speech before the United Nations Human Rights Council in Geneva in 2019, Mr. Mohib Ullah tried to describe all the ways in which the Rohingya were denied their humanity, from their citizenship to their very name. He was cut off after two minutes under council rules.

“Imagine you have no identity, no ethnicity, no country, nobody wants you,” he said in Geneva. “How do you feel? This is how we feel today as Rohingya.”

He visited the White House that same year and met President Donald J. Trump as part of a gathering of persecuted religious minorities from all over the world. Although he could have tried to claim asylum while in the United States or Europe, Mr. Mohib Ullah instead returned to the refugee camp, with its filthy latrines, crowded shelters and deadly landslides and fires.

“We are deeply saddened and disturbed by the murder of Rohingya Muslim advocate and community leader Mohib Ullah,” Antony J. Blinken, the United States secretary of state, said in a statement. “We will honor his work by continuing to advocate for Rohingya and lift up the voices of members of the community in decisions about their future.”

Mohammed Mohib Ullah was born to Fazal Ahmed and Ummel Fazal in a village in Maungdaw Township, a Rohingya-majority sliver of land abutting Bangladesh. His father was a teacher, and Mr. Mohib Ullah followed in his footsteps, teaching science. He was part of a generation of middle-class Rohingya who could still take part in Myanmar life. He studied botany at a college in Yangon, the country’s largest city, which is home to a sizable Muslim population.

In Maungdaw, a bustling town of markets and mosques, he took another job as an administrator. The work earned him the skepticism of some in the Rohingya community, who wondered if he was collaborating with the state oppressors. He countered that progress could come only through some sort of engagement.

In August 2017, Rohingya militants from the Arakan Rohingya Salvation Army attacked police posts and a military base in Rakhine State, killing about a dozen security forces. The response, girded by a troop surge in Rakhine weeks before, was ferocious. Soldiers, sometimes abetted by civilian mobs, rampaged through Rohingya villages, shooting children and raping women. Entire communities were burned to the ground. A United Nations human rights chief called it a “textbook case of ethnic cleansing.”

More than 750,000 Rohingya fled their homes in a matter of months, deluging Bangladesh. Mr. Mohib Ullah, his wife, Naseema Begum, and their nine children were among them. (His wife and children survive him.) As plan after plan for repatriation fizzled, he continued to call for both Bangladesh and Myanmar, along with the United Nations, to try harder. He missed Myanmar.

“We want to return home, but with dignity and safety,” Mr. Mohib Ullah said.

In the refugee camps, discontent simmered. Joblessness surged. The Bangladeshi government moved forward with a plan to relocate some Rohingya to a cyclone-prone silt island that some consider unfit for habitation. Security forces unrolled spools of barbed wire to confine the camps. ARSA militants searched for new recruits. Drug cartels canvassed for willing runners. Families worried that their little girls or boys would be kidnapped as child brides or servants.

Mr. Mohib Ullah spoke out against ARSA militancy, illicit networks and the dehumanizing treatment by Bangladeshi officialdom. For his safety, he sometimes had to be hidden in safe houses in Cox’s Bazar, the nearest city to the camps.

In the days leading up to his death. Mr. Mohib Ullah received more death threats, members of his family said. His brother, Mr. Habib Ullah, accused ARSA of orchestrating the assassination. He said he knew some of the gunmen and recounted how they had said that Mr. Mohib Ullah was not the leader of the Rohingya, they were. In a statement, ARSA said “transnational border-based criminals” were responsible for the killing.

Moderate camp leaders are running scared. Most are unwilling to speak on the record. They have switched off their phones. Mr. Mohib Ullah would likely have spoken up. That was his way. It was also, his friends say, what led to his death.