"Like Homero Gomez, Raul dedicated his life to the protection of the monarch butterfly in Michoacan," tweeted Amnesty International Mexico in Spanish on Monday. "We demand the immediate intervention of the authorities!"
The global human rights group's U.K. office added that the news of Hernandez's death was "horrific" and pledged to investigate.
Hernandez Romero's death, "along with the death of Homero Gomez, demands immediate investigation and full accountability," tweeted Richard Pearshouse, head of crisis and environment at Amnesty.
El Rosario sanctuary provides a home for millions of migrating monarch butterflies each year and draws thousands of tourists annually. But the reserve has also drawn the ire of illegal loggers in Mexico, who are banned from cutting down trees in the protected area.
Before the ban, more than 1,000 acres of the woodland were lost to the industry between 2005 and 2006.
Hernandez's family told the BBC that before he disappeared on Jan. 27, he had been receiving threats warning him to stop campaigning against illegal logging. Forensic experts said the activist appeared to have been beaten with a sharp object and had a deep wound in his head.
Mexico's murder rate has gone up in recent years, with more than 34,000 people killed in 2019--the highest annual number ever in the country. Between 2010 and 2016, fewer than 6% of murder cases led to convictions. In 2018, at least 14 eco-defenders were killed in Mexico, according to a report by Global Witness.
Last year, Amnesty International's Americas director Erika Guevara-Rosas told Mexico News Daily that "the brave defenders of the land, territory, and environment face constant danger in Mexico."
Guevara-Rosas demanded a thorough investigation into the deaths of Hernandez and Gomez on social media Monday.
"Raul, like Homero, had also denounced illegal logging," she tweeted. "Their deaths cannot go unpunished."
