Honduran indigenous activist Berta Caceres, an award-winning environmentalist, was murdered in her home Thursday, her family said, labeling her death an assassination.

Caceres's mother, Berta Flores, said police had indicated her daughter was killed in a robbery, "but we all know it was because of her struggle."

Caceres, a 43-year-old mother of four who had received death threats for her activism, was shot dead in the early hours of Thursday at her home in the western town of La Esperanza, Flores told TV network Globo.

Caceres won the 2015 Goldman Prize, considered the world's top award for grassroots environmental activism, for leading the indigenous Lenca people in a struggle against a hydroelectric dam project that would flood large areas of native lands and cut off water supplies to hundreds.

In awarding her the prize, the organization commended her for carrying on her campaign despite the threats, writing: "Her murder would not surprise her colleagues, who keep a eulogy -- but hope to never have to use it. Despite these risks, she maintains a public presence in order to continue her work."

President Juan Orlando Hernandez called the killing "a crime against Honduras" and vowed to bring the perpetrators to justice, while the secretary general of the Organization of American States, Luis Almagro, condemned the crime as "horrific."

Honduras, which has seen an explosion of gang violence in recent years, has one of the highest murder rates in the world.

Security Minister Julian Pacheco said police arrested two people, a security guard at the complex where Caceres lived and another suspect who was wounded, but he did not give further details on the investigation.

He said police had measures in place to protect Caceres, who recently won a ruling by the Inter-American Commission on Human Rights granting her special security measures.

Police formerly provided her an around-the-clock guard, but switched to an occasional security detail at the activist's own request, he added.

- 'Deficient' security -

But the Center for Justice and International Law, a rights group, denied Caceres had turned down bodyguards and accused the government of providing her "deficient" security.

The security minister said Caceres had spent the night away from the home that was registered with the authorities. Fellow activists said she had moved to a safe house fearing for her life.

Labor leader Carlos Reyes joined Caceres's mother in insisting that she was not just another victim of violent crime.

"The information from the police is that (attackers) broke into her home from the back and shot her twice, but we all know it's a lie, that they killed her because of her struggle," said Reyes.

"It's a political crime by the government."

Flores said her daughter had recently had a "very big altercation" with soldiers and representatives of a hydroelectric company during a visit to the Gualcarque river, where the company is at work on a dam project.

Caceres co-founded the Civic Council of Indigenous and People's Organizations (Copinh) in 1993 with her then husband Salvador Zuniga, gaining fame for her fearless fight against environmental destruction by hydroelectric and mining companies.

A diminutive woman with a round face, glowing eyes and bursts of curly black hair, she was active in a range of political and social causes.

But she was best known for her battle to save the Gualcarque river, which earned her the Goldman Prize last year -- dubbed by some the "Green Nobel."

On accepting the prize, Caceres linked her environmentalism to her indigenous roots.

"In our cosmic vision we are beings born of the Earth, the water and the maize plant. We are the ancestral custodians of the rivers," she said.