Mohibullah, who was known by one name and was in his late 40s, was killed by unidentified gunmen on Wednesday in the Kutupalong camp in Cox's Bazar.
On Thursday evening, thousands of refugees attended his funeral prayers before Mohibullah was buried in a graveyard inside the camp.
Police in Bangladesh said Friday they arrested a man in connection with the killing. The suspect also was a Rohingya, said Naimul Haque, a senior official with the Armed Police Battalion. He gave no other details and said he was being questioned.
The incident comes amid worsening violence at the sprawling settlement, which is home to hundreds of thousands of Rohingya refugees driven from neighboring Myanmar in 2017 by waves of state-sponsored violence.
Mohibullah had been an international advocate for Rohingya rights, including traveling to the White House for a meeting on religious freedom in 2019.
Secretary of State Antony Blinken said the U.S. was saddened by the murder and praised Mohibullah as a brave and fierce advocate for Rohingya rights.
“We urge a full and transparent investigation into his death with the goal of holding the perpetrators of this heinous crime accountable. 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,” Blinken said in a statement Thursday.
Mohibullah's death highlights the enduring struggles of the Rohingya, a Muslim minority group long persecuted by the Myanmar government, as they contend with not only displacement but threats to their safety within the camps, Human Rights Watch said.
“He always defended the rights of the Rohingya to safe and dignified returns and to have a say in the decisions concerning their lives and future,” Meenakshi Ganguly, the group's South Asia director, said in a statement.
“His killing is a stark demonstration of the risks faced by those in the camps who speak up for freedom and against violence.”
The U.N. refugee agency condemned the attack and said it was in contact with agencies responsible for the security of Rohingya refugees.
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"We urge the Bangladesh authorities to undertake an immediate investigation, and hold those responsible to account," it said.
As leader of the Arakan Rohingya Society for Peace and Human Rights, Mohibullah painstakingly documented the testimonials of survivors in the camps. The detailed record he provided has been cited in multiple investigations, including one by the International Criminal Court.
He addressed the U.N. Human Rights Council in 2019, and later that year he was part of a delegation of victims of religious persecution who met with former President Donald Trump at the White House.
In his remarks to the U.N. rights council, Mohibullah said the Rohingya had faced "systematic genocide" in Myanmar, where the government denies them citizenship.
"Imagine you have no identity, no ethnicity, no country, nobody wants you," he said. "How would you feel? This is how we feel today as Rohingya."
He said the Rohingya wanted to return home if they could be guaranteed citizenship and security.
"We are not stateless. Stop calling us that," he said. "We have a state, it is Myanmar."
But as Mohibullah's international profile increased, so did the number of death threats.
“Everyone in governments around the world who facilitated his travel overseas was well aware of the death threats he received, over the last 12 months in particular,” Eva Buzo, Mohibullah's lawyer, said in a phone interview with NBC News.
Saad Hammadi, South Asia campaigner for Amnesty International, said Mohibullah's killing "sends a chilling effect across the entire community."
The rights group said that since last year, at least 2,000 Rohingya refugees had been forced to flee their shelters by violence between two factions vying for control of the camp's illicit drug trade.