Tegucigalpa, Honduras(CNN) When masked attackers killed prominent Honduran activist Berta Cáceres, he was the only witness.
Now Honduran authorities say Mexican environmentalist Gustavo Castro can't leave the country for the next 30 days -- an order that's sparked an outcry from activists who fear he could be in danger.
Castro's brother accused investigators of trying to incriminate him, rather than protect him.
"They are trying to accuse my brother of a crime, that is to say, he's changed from being a witness to being someone who committed a crime, and they are trying to find a crime to pin on him however they can," Oscar Castro told reporters Wednesday.
At the press conference in the Honduran capital of Tegucigalpa, activists said Gustavo Castro's human rights had been violated.
"We observe with great concern that they have not given him the appropriate treatment. He has repeatedly said that he has cooperated with authorities and could keep cooperating from his country, but nonetheless they have arbitrarily blocked him from leaving the country," said Ana Marcia Aguiluz of the Center for Justice and International Law.
The public prosecutor's office denied that Castro's rights had been violated. He needs to stay in Honduras, officials said, so authorities can investigate Cáceres' slaying.
"He is a witness for prosecutors because, let's remember, he was at the scene when Mrs. Berta Cáceres died, so he is cooperating with prosecutors with his testimony," said Yuri Mora, a spokesman for the prosecutor.
In an open letter published by Honduran media earlier this week, Castro said he feared for his life.
"The hit men already know that I did not die, and they are surely ready to complete their assignment," he wrote, according to a copy of the letter posted on the website of CNN affiliate Televicentro.
Michel Forst, the United Nations special rapporteur on the situation of human rights defenders, criticized the way authorities were handling the case.
"Gustavo should immediately be provided with effective protection and permitted to return to his country," he said Thursday.
A member of the Lenca indigenous group, Caceres was one of her country's best-known activists, campaigning for indigenous and environmental causes. Last year she was awarded the prestigious Goldman Environmental Prize for her work in leading a successful campaign against one of Central America's biggest hydropower projects, the Agua Zarca Dam on the Gualcarque River.
Before she was killed, Cáceres had long been subject to repeated threats and harassment. She told CNN en Español last year that her country's government was often to blame for the targeting of indigenous leaders and activists.
Forst joined the chorus of human rights groups asking the Honduran government to allow an independent investigation into Cáceres' death.
In a letter Thursday, a group of more than 200 organizations asked for U.S. Secretary of State John Kerry's support and calling for the U.S. government to suspend training to Honduran security forces "so long as the murders of Berta Cáceres and scores of other Honduran activists remain in impunity."
Honduran authorities have rebuffed the notion, saying an independent inquiry is not necessary. National Police spokesman Hector Ivan Mejia said five police teams were investigating the case, and results were expected soon.
Mexico's foreign ministry said in a statement that it had asked Honduran authorities to allow Castro's return as soon as possible.
The Honduran government, according to the ministry, "has assured that it is taking all the necessary measures to protect the life and safety of the witness."