A security video recording shows masked men pulling up in a car in front of the governor's office in the 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 Thursday, said Jorge Gonzalez, special state prosecutor for crime prevention.
Escobedo was taken by ambulance to a hospital, where she died within minutes.
And far to the north in Ciudad Juarez, where Escobedo's 17-year-old daughter's burned and dismembered remains were found in a trash bin in June 2009, activists protested outside the state prosecutors office with signs demanding "Justice for Marisela."
Thursday's slaying "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, said Carlos Gonzalez, a spokesman for the attorney general's office in Chihuahua state, where Ciudad Juarez is located.
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 the case. Escobedo staged numerous marches, once wearing no clothes, wrapped only in a banner with her daughter's photograph.
"This struggle is not only for my daughter," Escobedo said through a megaphone at that march, her voice breaking. "Let's not allow one more young woman to be killed in this city."
Three days ago, she planted herself in front of the offices of Gov. Cesar Duarte and vowed not to move until investigators showed progress in the case. In an interview with the newspaper El Diario on Sunday, Escobedo said she had received death threats from Barraza's family.
Duarte said state security officials had been assigned to guard Escobedo, although from a distance. He said their failure to protect her Thursday would be investigated.
Duarte had also called on the state's top court to suspend the three judges.
On Friday, court president Javier Ramirez Benitez said they would be suspended pending an investigation. Ramirez Benitez said an oversight commission found earlier this year that the case was improperly handled.
Prosecutors said Barraza, Frayre's live-in boyfriend, admitted murdering her and led police to the body. But at trial he proclaimed his innocence and claimed he had been tortured into confessing. The judges ruled in April that prosecutors failed to present material evidence against him.
The case exemplifies the problems of the judicial system in Chihuahua state, one of the first to adopt oral trials instead of the closed-door interrogations and filings of documents used for most Mexican trials.
Despite training, Chihuahua police and prosecutors have struggled to adapt to a system that puts the burden of proof on prosecutors. Many homicide cases have been thrown out for lack of evidence or never make it to trial.
Police in Ciudad Juarez have been overwhelmed by drug gang battles that have made the city one of the world's deadliest. More than 3,000 people have been killed in the city of 1.3 million this year alone.
Records obtained by The Associated Press show that last year, when 2,600 people were killed in Ciudad Juarez, prosecutors filed 93 homicide cases and got 19 convictions.
Chihuahua's judicial deficiencies go back years before the new system was implemented, before drug violence soared to unprecedented levels.
In the 1990s, hundreds of women were killed around Ciudad Juarez, about 100 of whom were sexually assaulted and dumped in the desert.