Leading gay rights activist Xulhaz Mannan and his friend were found dead in an apartment in the Bangladeshi capital Dhaka on Monday.
Police said five or six men had entered the seven-storey building under the guise of delivering a package and attacked the pair with machetes.
According to witnesses, the unidentified assailants shouted "Allahu Akbar" and fired blanks as they fled the scene.
Mr Mannan launched Bangladesh's first magazine for the lesbian, gay, bisexual and transgender community, Roopbaan, with friends two years ago.
He had also worked for the US Agency for International Development and as a protocol officer for a former US ambassador to Bangladesh.
He helped organise an annual Rainbow Rally in Bangladesh, which was due to take place on 14 April, but was banned on security grounds.
Ahead of the event, Mr Mannan told the AFP news agency that the organisers had received threats from Islamist extremists.
Students held a protest over the killing of Rezaul Karim Siddique
"They have even set up an online group to threaten us," he said.
In a statement, US ambassador to Bangladesh Marcia Bernicat said: "I am devastated by the brutal murder of Xulhaz Mannan and another young Bangladeshi.
"We abhor this senseless act of violence and urge the Government of Bangladesh in the strongest terms to apprehend the criminals behind these murders."
The second victim has been identified as Mr Mannan's friend, Tanay Majumder.
A security guard was also injured in the attack and is undergoing treatment at Dhaka Medical College Hospital.
The killings are the latest in a series of brutal attacks targeting minorities, liberal activists and secular bloggers in the Muslim-majority nation.
They come two days after a liberal professor, Rezaul Karim Siddique, was hacked to death in the northwestern city of Rajshahi.
The Islamic State group claimed responsibility for the attack through its news agency, saying the 58-year-old professor had been slain for "calling for atheism".
The local branch of al Qaeda, meanwhile, has claimed the murders of numerous secular bloggers and activists, including the killing this month of a 26-year-old atheist law student who mocked Islam on his Facebook page.
The government has denied that Islamic State or al Qaeda groups have a presence in Bangladesh, claiming that homegrown Islamist radicals are behind the recent attacks