More than 50 humanitarian and environmental groups from around the world called on Friday for an independent international investigation into the assassination of Honduran Indigenous rights activist Berta Cáceres, who was murdered in her sleep at 1am on Thursday by two unknown assailants.

"Mrs. Cáceres' case is the most high-profile killing within a growing trend in the murder, violence, and intimidation of people defending their indigenous land rights in Honduras," wrote the groups in their letter to the Honduran president.

"We know that in Honduras it is very easy to pay people to commit murders," Zuñiga Caceres said of her mother’s death to teleSUR. "But we know that those behind this are other powerful people with money and a whole apparatus that allows them to commit these crimes."

Cáceres was a prominent leader in the Indigenous movement in Honduras against one of Central America’s largest hydropower projects, four enormous dams known as "Agua Zarca" in the Gualcarque river basin, the Guardian reported. The Indigenous group Cáceres founded, Civil Council for Popular and Indigenous Organizations of Honduras (COPINH), has so far been successful in preventing the project from moving forward.

Cáceres was awarded the prestigious Goldman Environmental Prize for her activism just last year.

"Berta Cáceres devoted her life to protecting natural resources, public spaces, land rights, rivers from the privatization process that’s underway and that gained speed after the 2009 military coup," said Karen Spring, the Honduras-based coordinator of the social justice network Honduras Solidarity Group, in an interview with Free Speech Radio News on Thursday. "She spent her life defending land and and basically supporting communities, mostly indigenous communities all over the country."

As a result of her activism, Cáceres had received death threats and feared for her life, the Los Angeles Times reported, and the Inter-American Commission on Human Rights (IACHR), a prominent human rights organization, had last year formally called on the Honduran government to put protections in place for Cáceres, according to the Guardian. The UN has condemned the Honduras government for failing to protect her, and activists have accused the government of having a hand in her death.

In its most recent report (pdf) released in December, IACHR warned of the violence and threats to their lives that activists such as Cáceres suffer under in Honduras. The group blamed "the increased presence of organized crime and drug traffickers, the recruitment of children and adolescents, and an inadequate judicial response that fuels impunity, corruption, and high levels of poverty and inequality. In addition, according to the information received, part of that insecurity comes from the National Police, the Military Police, and the Army, through their illegitimate use of force, in some cases in complicity with organized crime.”

Student protesters took to the streets in Tegucigalpa on Thursday to mourn the widely beloved environmentalist's death, the Guardian reported, and the Honduran government, in power since a U.S.-backed coup in 2009, responded with riot police.

One suspect has been arrested, the Honduras government confirmed in a statement to teleSUR on Friday. There were reportedly two assassins involved in Cáceres' death. But the Cáceres family is demanding "an independent, international investigation [into her death] not led by the Honduran government," teleSUR reported.

"Honduras has the world’s highest murder rate," noted School of the Americas Watch, a group that seeks to close the infamous U.S. Army School of the Americas, in a statement on Thursday. "Honduran human rights organizations report there have been over 10,000 human rights violations by state security forces and impunity is the norm—most murders go unpunished. The Associated Press has repeatedly exposed ties between the Honduran police and death squads, while U.S. military training and aid for the Honduran security forces continues."

The environmental group International Rivers demanded Thursday that "the U.S, government, in particular, end its support for the Honduran military through loans and through training at the School of the Americas," drawing attention to the United States' significant responsibility for the oppressive regime in Honduras today, in order "to honor Berta Cáceres' lifelong struggle and her ultimate sacrifice for rivers and rights."

Democracy Now! remembered Cáceres in the following segment on Thursday: