DHAKA, Bangladesh (AP) — Rights groups, the United Nations and the U.S. government have called for a full investigation into the killing of a key Rohingya leader who was shot to death in a refugee camp in Bangladesh.

Mohibullah, who was known by one name, had been an international advocate for Rohingya rights, including traveling to the White House for a meeting on religious freedom in 2019.

Police said he was shot by unknown attackers late Wednesday at the Kutupalong refugee camp at Ukhiya in Cox’s Bazar district in southern Bangladesh. No group has claimed responsibility.

He was the chairman of the Arakan Rohingya Society for Peace and Human Rights and was known as a voice for starting the repatriation of the refugees to Myanmar.

A colleague at the society said Mohibullah was killed in his office in front of several witnesses.

“Suddenly a group of eight to 10 people entered the office and three of them surrounded Mohibullah from three sides and rest were controlling the other elderly people present there who were very old. One pointed the gun between his eyes, the other on his chest and another one here on the arm and they all fired. Then they fired two more shots in the air and hurriedly fled. No one even realized as all this happened so fast,” said Mohammed Sharif, 60, his co-worker at the society.

Fear and frustration gripped the sprawling camps after the killing.

“We Rohingya have now become like cattle ready for slaughter, what else can I say. I am still in shock. The people who had targeted my nephew are the same people who killed him and made him a martyr. And that is the cold truth,” said Sayed Alam, an uncle of Mohibullah.

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.

The U.N High Commission for Refugees condemned the killing, expressed “deep shock and sadness" and called for the government to do its best to protect the refugees.

“We extend our deepest condolences to Mohib Ullah’s family, and to the wider Rohingya refugee community who are mourning his loss,” the agency said in a statement.

On Thursday evening, thousands of refugees attended his funeral prayers before Mohibullah was buried in a graveyard inside the camp.

Mohibullah, a former teacher who was in his 40s, had served as a spokesman representing the Muslim ethnic group in international meetings. He visited the White House in 2019 for a meeting on religious freedom with then-President Donald Trump and spoke about the suffering and persecution faced by Rohingya in Myanmar.

That same year, he was bitterly criticized by Bangladeshi media after he led a massive rally of 200,000 refugees to mark the second anniversary of the crackdown by Myanmar’s military that caused about 700,000 Rohingya, including Mohibullah, to flee to neighboring Bangladesh.

Human Rights Watch called Mohibullah a vital voice for the Rohingya community. “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. His killing is a stark demonstration of the risks faced by those in the camps who speak up for freedom and against violence,” Meenakshi Ganguly, the rights group's South Asia director, said in a statement.

“Mohibullah’s death undermines not only the struggle of Rohingya refugees for greater rights and protection in the refugee camps, but also their efforts to safely return to their homes in Myanmar. Bangladesh authorities should urgently investigate Mohibullah’s killing along with other attacks on Rohingya activists in the camps," she said.

Amnesty International condemned the killing and urged Bangladeshi authorities and the U.N. refugee agency to work together to ensure the protection of people in the camps, including refugees, activists and humanitarian workers from both the Rohingya and local community, many of whom have shared concerns about their safety.

“Violence in the refugee camps in Cox’s Bazaar has been a growing problem," said Saad Hammadi, Amnesty International’s South Asia Campaigner. "Armed groups operating drug cartels have killed people and held hostages. The authorities must take immediate action to prevent further bloodshed."

Overall, Bangladesh has been sheltering more than 1.1 million Rohingya refugees from Myanmar after previous waves of persecution.