White House Press Secretary Josh Earnest took note of Mannan's advocacy for lesbian, gay, bisexual and transgender causes and said there were "reports that indicate that he was targeted because of his advocacy for these human rights and that makes his death even more tragic than it seems."
"At this point, our expectation is that the government of Bangladesh should engage in a serious criminal investigation to determine who was responsible and to bring those individuals to justice. They have committed a heinous crime and they should be held accountable for it," Earnest told reporters at his daily news conference.
Earnest said Mannan served the US embassy in Dhaka with distinction and "he worked on behalf of his fellow Bangladeshis as a voice for justice, equality of human rights for all, including for the local LGBT community."
"While his death is obviously a significant tragedy, there are also reports that indicate that he was targeted because of his advocacy for these human rights, and that makes his death even more tragic than it seems," he said.
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"We strongly urge the government of Bangladesh to ensure that the perpetrators of this senseless crime are brought to justice," he said.
Mannan, the editor of 'Roopban' - the only magazine in Bangladesh advocating gay rights - and his friend Tanay Majumder were killed yesterday by armed assailants in his flat in Dhaka impersonating as courier company officials.
The Al-Qaeda in the Indian Sub continent (AQIS) has claimed responsibility for killing the duo, saying that the two were because they were "pioneers of promoting and practicing homosexuality.