Comunicando y Capacitando Trans (Comcavis Trans) said that Diaz had disappeared on the night of Nov. 6 and her body was discovered on Saturday morning.
“We demand that the government carry out an immediate and thorough investigation of the incident,” the group said in a statement.
It also urged the government of El Salvador to take “urgent measures to ensure the safety” of human rights defenders and members of the LGBT community.
Comcavis Trans director Bianka Rodriguez said last month in an interview with EFE that at least 20 trans women had been killed in El Salvador between 2017 and October 2019.
She added that the victims were aged between 16 and 32 years and five trans women had already been murdered this year.
The killings recorded this year include those of Anahi Rivas and Camila Diaz.
Diaz was detained on Jan. 31 by three police officers, was reportedly repeatedly hit during her detention and she died three days later due to injuries.
Rivas was kidnapped, tortured and killed in San Salvador, with the crime described by LGBT organizations as femicide.
The violence and discrimination faced by the trans community in El Salvador has forced many of its members to flee the country in order to escape violence from criminal gangs or state security forces, which have been accused of being complicit in the killings and impunity.
Data provided to EFE by Comcavis Trans in August said that between 2018 and 2019, 141 cases of forced displacement of LGBTQ persons were reported in the country.
Trans women formed 67.5 percent of the cases, followed by gay men, who figured in 17.2 percent of the incidents.