In Mexico, 53 local police officers are being questioned over the disappearance of environmental activist Homero Gómez.
Mr Gómez, who manages a butterfly sanctuary in the central town of Ocampo, was last seen on 13 January.
Prosecutors said they had detained the entire police forces of Ocampo and neighbouring Angangueo for questioning.
Prosecutors have not said why they suspect the local police officers of involvement in Mr Gómez's disappearance.
It is not the first time the police force of Ocampo has been detained for questioning. In June 2018, 27 officers were arrested on suspicion of involvement in the murder of a local politician who was running for mayor.
Ocampo is located in Michoacán state, a region that is notorious for its violent criminal gangs, which often bribe or intimidate local police into working for them.
Relatives of Mr Gómez told local media that the conservationist had received threats from an organised crime gang.
Rights groups had earlier said they feared that Mr Gómez may have been targeted because of his fight against illegal logging, one of the activities that criminal gangs in the area are involved in.
Mr Gómez is a tireless campaigner for the conservation of the monarch butterfly and the pine and fir forests where it hibernates.
Scientists have been fascinated with the orange and black monarch butterflies because of the vast distances they fly each year to hibernate in Mexico - the longest migration of any insect.
The sanctuary Mr Gómez manages near Ocampo opened in November as part of a strategy to stop illegal logging in the area, which is a key habitat for the monarch butterfly.
In a video posted on his Twitter account shortly before his disappearance, he invited people to visit the sanctuary.
En el Santuario El Rosario Ocampo Michoacan “ El más grande del mundo “ pic.twitter.com/WlCJuOcG4Q— Homero gomez g. (@Homerogomez_g) January 12, 2020
He was last seen in person attending a meeting in the village of El Soldado on the afternoon of 13 January and his family reported him missing the next day.
A search which was joined by 200 volunteers has so far yielded no clues. Since 2006, 60,000 people have disappeared in Mexico, many of them are believed to have fallen victim to criminal gangs who kill anyone who could interfere with their illegal activities.