The Pakistani authorities should conduct a prompt and impartial investigation into the killing of human rights activist Khurram Zaki and appropriately prosecute those responsible, Human Rights Watch said on Tuesday.
Zaki, 40, had been publicly critical of extremist cleric Abdul Aziz and militant sectarian groups.
On May 7, four unidentified gunmen opened fire at Zaki at a restaurant in Karachi, killing him and wounding two others.
Zaki had been receiving threats and had confided to friends that he was on several militant 'hit lists'.
A thorough and impartial investigation with proper witness protections is absolutely critical to ensure those responsible for Khurram Zaki's death are brought to justice, said Brad Adams, Asia director of the Human Rights Watch.
The Hakeemullah group, a relatively unknown faction of the Pakistani Taliban, claimed responsibility for the murder, saying that Zaki was targeted because of his stance against Abdul Aziz.
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However, the Pakistani authorities and others have expressed skepticism at this claim.
On May 9, the police registered a criminal case against Abdul Aziz, Arungzeb Farooqi, a leader of the anti-Shia Ahle-Sunnt-Wal-Jammat (ASWJ), and the unidentified gunmen.
Abdul Aziz has been acquitted in several criminal cases in the past because witnesses recanted their statements.
The authorities have accused Aziz of abducting Chinese nationals working in Pakistan, inciting supporters to commit violence against state functionaries, targeting security officials, and extending support to the extremist group Islamic State (ISIS).
The Pakistan Government has failed to take a clear stand against intimidation and violence by militant groups or defend the right to freedom of expression, the Human Rights Watch said.
The censorship of documentaries and critical comment on violence by sectarian groups makes a few people who express their opposition more vulnerable, it said.
"Zaki's murder highlights the unacceptably dangerous climate that human rights defenders face across Pakistan," Adams said.
Article 12 of the United Nations Declaration on Human Rights Defenders sets out that governments shall take all necessary measures to ensure the protection for human rights defenders against 'any violence, threats, retaliation, de facto or de jure adverse discrimination, pressure or any other arbitrary actions' related to their efforts to protect and promote human rights and fundamental freedoms.
"The Pakistan Government needs to protect rights activists and promote an environment where they can carry out their work free from threats, attacks and intimidation," Adams added.