A mother who waged a two-year battle to bring her daughter's killer to justice was shot to death, possibly by the man suspected of murdering the girl.

A security video shows masked men pulling up in a car in front of the governor's office in Mexico's northern city of Chihuahua. One appeared to exchange words with anti-crime crusader Marisela Escobedo Ortiz, who was holding a vigil outside. She tried to flee by running across the street, but the gunman chased her down and shot her in the head late on Thursday. She was taken by ambulance to a hospital, where she died within minutes.

On Friday, a group of demonstrators gathered outside the interior department in Mexico City to protest the killing, briefly scuffling with police while chanting "Not one more death".

Far to the north in Ciudad Juarez, where Escobedo's 17-year-old daughter's burned and dismembered remains were found in a rubbish bin in June 2009, activists protested outside the state prosecutors office with signs demanding 'Justice for Marisela'.

Thursday's killing "shows that in Mexico it is the victim who suffers, without protection", veteran anti-crime activist Alejandro Marti said.

The scandal resulted in the suspension of three judges who had ordered the release of the main suspect in the daughter's killing after he was absolved by a court in April for lack of evidence. That man, Sergio Barraza, is now a chief suspect in the mother's death.

Escobedo's daughter, Rubi Frayre Escobedo, disappeared in Ciudad Juarez, across from El Paso, Texas, in 2008. After the body was discovered last year, the mother launched a campaign pressing for a conviction.

In an interview with the newspaper El Diario last weekend, Escobedo said she had received death threats from Barraza's family.