Local officials said Ms Valles had asked for time off work to care for her sick baby, but had not returned on Monday as expected. The mayor of Praxedis said attempts to contact her had been unsuccessful, and he would take over direct control of the police force.
Local human rights activists told that Ms Valles and her family had fled to the US after receiving threats. The criminology student attracted headlines around the world in October when she took on the job of head of public security in Praxedis after other possible candidates were reportedly too scared to apply.
At the time, she told the BBC she was accepting the role despite the risks involved because she felt Mexican citizens had a responsibility to try to improve security.
Praxedis is in the Juarez Valley in the northern state of Chihuahua, a battleground for drugs cartels fighting over lucrative smuggling routes across the US border.
Local police and officials in the region have been frequent targets for attack.
The town is close to Ciudad Juarez, Mexico's most violent city, where more than 3,000 people were killed in drug-related violence in 2010 alone.
Credits: BBC
