Two people, one of them a gay rights activist, were chopped to death by unidentified assailants in the Bangladeshi capital of Dhaka, police and media reports said on Monday.
At least five assailants dressed as courier service staff entered the apartment building of Xulhaz Mannan, an editor of a gay rights magazine, and stabbed him to death, police officer Rezaul Islam, said, quoting witnesses.
The assailants also killed Mannan's friend, Tonoy Majumder, and stabbed building security guards.
Mannan was an employee at the US embassy in Dhaka, the embassy said in a statement.
The US State Department said it was "outraged" by the killing and praised him as "a beloved member of our embassy family and a courageous advocate for LGBTI rights."
"An act like this simply is beyond words, unjustifiable, inexcusable and our heartfelt condolences of course go out to his mother, to his family, to his friends and to his colleagues, as well as all those who knew and loved the other individual who was also brutally murdered with him," spokesman John Kirby said.
Private broadcaster Somoy Television said Mannan was the editor of Roopbaan, Bangladesh's first magazine promoting the rights of gays, lesbians, bisexuals and transgender people.
Two bloggers, Niloy Neel and Avijit Roy, who had praised the magazine at the Muktomona blog site were killed in similar fashion by suspected Islamist militants in 2015.
An on-duty officer at Kalabangan police station, Habibur Rahman, told reporters that Mannan was a cousin of former Bangladeshi foreign minister Dipu Moni.
The attackers were in blue T-shirts and were carrying bags, Rahman said.
No group has yet claimed responsibility for the killings that came just two days after a university professor was hacked to death in the northern Bangladeshi city of Rajshahi.
The extremist militia organization Islamic State claimed responsibility for killing the professor.
US Ambassador Mercia Bernicat condemned Xulhaz's killing in a statement, asking the Bangladeshi government to arrest the criminals behind the murder.
"Xulhaz was more than a colleague to those of us fortunate to work with him at the US Embassy: He was a dear friend," the ambassador said.
"We abhor this senseless act of violence and urge the government of Bangladesh in the strongest terms to apprehend the criminals behind these murders."