This Philadelphia mother risked her life to find her son’s killer. Her story is part of a new documentary.

There is a moment in a new docuseries about mothers who risked their lives to get justice for their children when Lisa Espinosa looks into the camera and says to her son’s killer: “You messed with the wrong mother.”

Her declaration, part of the ABC News Studio series Mother Undercover, captures the Philadelphia mother’s dogged determination to hold her son’s murderer accountable.

On April 10, 2016, after a fight outside a nightclub at B Street and Allegheny Avenue in Kensington, 26-year-old Raymond Pantoja was fatally shot.

In a city where hundreds of people are killed each year, but only a fraction of murder cases are solved, Espinosa was intent on making sure her youngest son’s death didn’t become another statistic.

So when leads dried up and she stopped hearing from detectives, she became her own investigator — putting herself on a dangerous path that led to a violent drug dealer.

Espinosa mined social media for clues she shared with police. She took to the streets to stake out witnesses. When she got too close, she and her family were threatened. But Espinosa was undeterred. At one point, she even held an event for families of murder victims across the street from where she knew her son’s killer hung out.

“My obsession was bigger than my fear,” she said when we talked this week.

While many viewers have just been introduced to Espinosa, and three other mothers who went to great lengths for their children, her story should be a familiar one to readers of this column.

For years, I documented Espinosa’s search for justice as she became one of the city’s most vocal advocates for victims of gun violence. (Full disclosure: I also make a brief appearance in the docuseries.) Even while trying to solve her son’s murder, Espinosa guided other families looking for answers. She also organized a protest outside of Philadelphia Police Department headquarters to highlight the city’s staggering number of unsolved murders.

Three years after her son’s death, Espinosa finally faced his killer, who pleaded guilty and was sentenced to 24 to 28 years in prison. The bullet that killed her son, she said in court that emotional day, also shattered the worlds of everyone who loved him — including his only daughter.

Afterward, family and friends told her she should go home and rest. All of her hard work, her courage had paid off. But just hours later, Espinosa stood before hundreds of people impacted by gun violence who had gathered at the Art Museum steps. Through tears, she said that no one should find comfort in a city where so many are living in anguish.

In many ways, Espinosa’s story is not unlike the stories of other families. But hers offers a blueprint for those hoping to keep the deaths of their loved ones from being eclipsed by the constant bloodshed in our city.

She hopes she can offer some inspiration. More so, she hopes that her work is a reminder that it is crucial to hold accountable every part of a system that fails families of homicide victims, and the institutional barriers that compound their grief. That stretches from the struggle to reach detectives to navigating a judicial system that often fails to put victims at the center of its efforts.

“The institutions that should put victims first are broken,” she said. “There are so many barriers that only add to our pain.”

By her count, she went to court more than 50 times as her son’s case slowly wound its way through the legal system. She all but begged some media outlets to share her story. She lost count of the multiple assistant district attorneys assigned to her case.

After the show premiered last month on Hulu, Espinosa was overwhelmed by people reaching out from all over the country. Many wanted her to know that her story touched them, but even more asked for guidance.

A mother in Kentucky. Another in Florida. Still another in Texas — all with their own stories of loss, all with the same plea: “Can you help?”

Espinosa is now a fellow for the national organization Everytown for Gun Safety and a volunteer for the nonprofit’s grassroots movement Moms Demand Action. (She’s hoping to become a senior fellow, which would allow her to mentor other survivors, so if you’ve been inspired by her story, text HONOR/Lisa Espinosa to 64433.) And lately, she’s begun to think more seriously about turning the lessons she learned into a book.

Lesson No. 1: Far too many families in Philadelphia are forced to put their grief on hold while desperately seeking justice that most never get.