Ms Miriam Rodríguez clutched a pistol in her purse as she ran past the morning crowds on the bridge to Texas.
She stopped every few minutes to catch her breath and study the photo of her next target: the florist.
She had been hunting him for a year, stalking him online, interrogating the criminals he worked with, and even befriending unwitting relatives for tips on his whereabouts.
Now she finally had one - a widow called to tell her that he was peddling flowers on the border.
Ever since 2014, she had been tracking the people responsible for the kidnapping and murder of her 20-year-old daughter, Karen.
Half of them were already in prison, not because the authorities had cracked the case, but because she had pursued them on her own, with a meticulous abandon.
She cut her hair, dyed it and disguised herself as a pollster, a health worker and an election official to get their names and addresses.
She invented excuses to meet their families, unsuspecting grandmothers and cousins who gave her details, however small.
She wrote everything down and stuffed it into her black computer bag, building her investigation and tracking them down, one by one.
She knew their habits, friends, hometowns, childhoods.
She knew the florist had sold flowers on the street before joining the Zeta cartel and getting involved in her daughter's kidnapping.
Now he was on the run and back to what he knew, selling roses to make ends meet.
Without showering, she threw a trench coat over her pyjamas, a baseball cap over her fire engine-red hair and a gun in her purse, heading for the border to find the florist.
On the bridge, she scoured the vendors for flower carts, but that day he was selling sunglasses instead.
When she finally found him, she got too excited, and too close. He recognised her and ran.
He sprinted along the narrow pedestrian pass, hoping to get away.
Ms Rodríguez, 56 at the time, grabbed him by the shirt and wrestled him to the rails. She jammed her handgun into his back.
"If you move, I'll shoot you," she told him, according to family members involved in her scramble to capture the florist that day.
She held him there for nearly an hour, awaiting police to make the arrest.
In three years, Ms Rodríguez captured nearly every living member of the crew that had abducted her daughter for ransom, a rogues' gallery of criminals who tried to start new lives - as a born-again Christian, a taxi driver, a car salesman, a babysitter.
In all, she was instrumental in taking down 10 people, a mad campaign for justice that made her famous, but vulnerable.
No one challenged organised crime, never mind put its members in prison. He asked the government for armed guards, fearing that the cartel had finally had enough.
On Mother's Day, 2017, weeks after she had chased down one of her last targets, she was shot in front of her home and killed.
Her husband, inside watching television, found her face down on the street, hand tucked inside her purse, next to her pistol.
For many in the northern city of San Fernando, her story represents so much of what is wrong in Mexico - and so remarkable about its people, their perseverance in the face of government indifference.
The country is so torn apart by violence and impunity that a grieving mother had to solve the disappearance of her daughter largely on her own, and died violently because of it.
Her stunning campaign - recounted in case files, witness testimony, confessions from the criminals she tracked down and dozens of interviews with relatives, police officers, friends, officials and local residents - changed San Fernando, for a while at least.
People took heart at her fight, and found indignation in her death.
The city placed a bronze plaque honouring her in the central plaza.
Her son, Luis, took over the group she had started, a collective of the many local families whose loved ones had disappeared. The authorities pledged to capture her killers.
