Honduran indigenous leader Berta Cáceres, who won the 2015 Goldman Environmental Prize for her role in fighting a dam project, was murdered Thursday.
Cáceres, a Lenca Indian activist, had previously complained of receiving death threats from police, soldiers and local landowners because of her work.
Tomás Membreno, a member of her group, the Indian Council of People's Organizations of Honduras, group said at least two assailants broke into a home and shot Caceres to death early Thursday in the town of La Esperanza.
"Honduras has lost a brave and committed social activist," Membreno said in a statement.
The killing appeared to be targeted: Mexican rights activist Gustavo Castro Soto at the house was only slightly wounded in the attack, but Cáceres's body had four gunshot wounds. Police said they had detained a suspect, but did not identify the person.
Cáceres, a mother of four, led opposition to a proposed dam on the Gualcarque river, considered sacred by the Lencas.
Many of the project's backers have largely abandoned building plans.
President Juan Orlando Hernández wrote in his Twitter account that "this act has caused mourning among all Hondurans."
His chief of staff, Jorge Alcerro, "The president has instructed all government security forces to use all means to find the killers."
Alcerro said Cáceres was supposed to be receiving special protection because of the death threats, but did not explain why there were no police protecting her when she was killed.
The U.S. ambassador in Honduras, James D. Nealon, issued a statement saying, "We strongly condemn this despicable crime. The United States of America calls for a prompt and thorough investigation into this crime and for the full force of the law to be brought to bear against those found responsible."
The United Nations special rapporteur on the rights of indigenous peoples, Victoria Tauli-Corpuz, wrote that "it is highly probable that her assassination is linked with her work in protecting the human rights of the Lenca indigenous peoples to their lands and territories."
The website of the Goldman Environmental Prize said Cáceres "waged a grassroots campaign that successfully pressured the world's largest dam builder to pull out of the Agua Zarca Dam," which the site said "would cut off the supply of water, food and medicine for hundreds of Lenca people and violate their right to sustainably manage and live off their land."
Erika Guevara-Rosas, Americas Director for Amnesty International, said in a statement that "the cowardly killing of Berta is a tragedy that was waiting to happen."
"For years, she had been the victim of a sustained campaign of harassment and threats to stop her from defending the rights of indigenous communities," said Guevara-Rosas. "Berta's death will have a devastating impact for many human rights activists and organizations."
Human Rights Minister Karla Cueva said "this crime cannot go unpunished."