MEXICO CITY  Marisol Valles García, the young mother who took the job of police chief in her violence-ravaged town when nobody else would, has not been to work in three days. On Friday, it appeared she might not be planning to come back soon.

A human rights official in Chihuahua State said that Ms. Valles might have been threatened and crossed into Texas. But her bosses said she asked for a few days of leave to attend to her baby son, who had been ill.

Gustavo de la Rosa Hickerson, who is in charge of the Ciudad Juárez office of the state human rights commission, said he had spoken to someone who had accompanied Ms. Valles to the border and watched her cross the bridge. He did not know if she had crossed alone or with family.

“We are trying to reach her to help her,” said Mr. de la Rosa, who said that he had not spoken to her and that he was trying to confirm reports that she had been threatened.