As police chief of a small Mexican town caught up the country's drug wars, Marisol Valles García hoped that turning a blind eye on the criminals would keep her safe. But just five months after taking up the post she is in hiding in the US, where she has applied for asylum.

"Her neutrality was not allowed," Gustavo De La Rosa, a local human rights ombudsman, said. "It sounds like this stemmed from some new capo taking over the territory and demanding absolute unconditionality."

A 20-year-old criminology student with a small baby, Valles became an international media sensation last October when she was named police chief of Praxedis G Guerrero in the Juárez valley, just over the border from Texas.

As a local police chief she had no legal obligation to deal with large-scale drugs crimes, which are primarily investigated by federal authorities. But she became a symbol of resistance to the view that community life was now impossible in much of Mexico.

Rival drug trafficking organisations have been fighting to control towns in the Juárez valley since 2008, unleashing a wave of violence which is even more intense than in the infamously murderous city of Ciudad Juárez, just to the north-west.

Valles' predecessor in Praxedis was kidnapped and decapitated in August 2009, and 15 of the 17 officers he commanded were also killed. The church was firebombed in April last year. Burnt out houses dotted around town provide permanent testament to the tragedy.

When she took the job, Valles appeared remarkably unflustered by the potential danger. "Of course I am frightened, I am only human," she said. "But you have to learn to trust and to have hope that things can change."

Her security plan focused on sending her eight unarmed female agents on door-to-door visits in an effort to persuade families to venture out of their houses again for community sports and cultural events. She also repeatedly insisted that the municipal police would be ignoring the organised criminals roaming the area.

How much this strategy of non-confrontation helped is not clear, but Praxedis had been notably calm in the last few months, unlike other towns in the area.

Then, at some point overnight between 2 and 3 March Valles and her immediate family disappeared, leaving a locked house with the lights still on.

Initial reports that they had crossed the border could not be confirmed and some worried they had been kidnapped. The municipal authorities revealed that Valles had taken a few days off to attend to her sick baby, and insisted they knew nothing of any threats to her. When she did not return to work on Monday as arranged, the mayor fired her.

"She lost her job because she abandoned it," Andrés Morales, a municipal spokesman, said. "Everything remains calm here and the rest of the police are working as normal."

The US authorities finally confirmed on Tuesday that Valles had been held in a detention centre for migrants in El Paso, just across the Rio Bravo from Ciudad Juárez. She was released, pending asylum hearings after an interview established her "credible fear".

Later that day, Valles called De la Rosa. "She said she was in a city far from the border and that we shouldn't worry any more," he said. "She asked me to tone things down."

Valles' flight after receiving serious threats and her consequent failure to beat the odds underline the vulnerability of anyone who takes on a public role in the Juárez area.

Last week, some 32 members of the a family of prominent human rights activist abandoned their homes in Guadalupe, also in the Juárez valley. In recent months a long family tradition of activism on a range of human rights issues had boiled down to the demand for justice in the cases of six members of the family who were murdered in several attacks over the last two years. Most of the family is now under police protection in Mexico City.