Authorities in Mexico are investigating the mysterious disappearance of a well-known conservationist in the state of Michoacan, as some human rights activists suggest his disappearance could be linked to illegal logging in the area.

Homero Gomez Gonzalez, one of Mexico’s foremost experts on monarch butterflies who has spent years protecting the insects that migrate to the area each winter from Canada and the U.S., was last seen on Jan. 13 while attending a meeting in a nearby town.

An official with the Human Rights State Commission of Michoacan told Reuters the organization has asked the attorney general’s office to look into whether Gomez Gonzalez’s disappearance is related to his conservation work, as he has been vocal in the past about the threat of illegal logging to the monarch butterfly’s habitat.

“He was probably hurting the [business] interests of people illegally logging in the area,” commission official Mayte Cardona told Reuters. Cardona said more than 200 volunteers have joined in the search for Gomez Gonzalez since last week.

Reuters reports a source with the state prosecutor says the investigation is ongoing.

Gomez Gonzalez manages the El Rosario Butterfly Sanctuary in Michoacan, a state notoriously known for its violent criminal activity. The day he went missing, he had tweeted two videos of thousands of monarchs searching for water in the sanctuary.

The El Rosario sanctuary was opened in November 2017 in an effort to curb illegal logging in the monarch’s habitat.

In 2018, the World Wildlife Fund reported that forest degradation in the region had decreased dramatically, but said illegal logging was on the rise.

Authorities reported this month that more than 61,000 people have disappeared in Mexico, while more than 97 percent of those disappearances have been reported since 2006.

The Municipal Police, Michoacan’s public-security ministry, and the state’s common land authorities are involved in the search for Gomez Gonzalez.