For Antonio Tizapa, the New York City marathon is more than a race. It is how he keeps his body healthy to heal his injured soul. Tizapa began running the marathon three years ago after his 19-year-old son Jorge Antonio Tizapa Leguideño disappeared in Guerrero, Mexico – one of the 43 students abducted by Mexican authorities on 26 September 2014 and never heard from again.

The mystery of what happened to the students from the Ayotzinapa school for teachers – who were disappeared after commandeering several buses to go to a protest in Mexico City – remains unsolved. Investigations by journalists and human rights researchers suggest a massacre was carried out by the Mexican army and police linked to drug traffickers.

While the scandal of their disappearance caused protests and political upheaval in Mexico, Tizapa experienced the agony from a distance. Living in New York illegally since 2000, Tizapa, now 50, felt helpless and unable to return since his salary as a plumber in Brooklyn has been the sole support for his wife, his two other children and the baby his son left behind when he disappear.

The parents of the other missing “43”, as they became known, marched in Mexico City to demand answers from the government and Tizapa took part in protests outside the Mexican consulate in Manhattan. Before the tragedy, he had started running casually and participated in a few short races, but the more he thought about it he began to see the New York marathon as a powerful platform to publicize the plight of the missing students. So he stepped up his training routine.

With the help of social media, he spread the word about his race. On the day of his first marathon in 2015 he donned a handmade t-shirt with the inscription: My son is your son and your son is my son: Ayotzinapa 43. Dozens of volunteers held signs with pictures of the missing students along the 26.2-mile route. His race, in three hours and 44 minutes, garnered media attention. Soon more runners were seeking him out to join his call to action and started a group called “Running for Ayotzinapa”. This year, he will run with at least 30 other marathoners all wearing shirts to push for answers about Ayotzinapa.

“A lot of people think running a marathon is about your time or ranking but for me it is about creating awareness,” Tizapa said during a series interviews at his training sessions in Brooklyn’s Prospect Park, “so that this never happens again in our country.”

Running doesn’t make me forget, but it makes me feel stronger Antonio Tizapa

As the years go by with no clearer answers about what happened to his son, the marathon has done more than given him a platform to speak out: it has helped him cope with his loss.

“The park has helped me a lot,” Tizapa said. “Running doesn’t make me forget, but it makes me feel stronger. I don’t know what I would do if I stayed home in my apartment.”

He added: “When I started running it was just a hobby. After everything happened with my son, I used the sport as a form of silent protest. But now over time also it has become a form of therapy.”

According to the official investigation of that terrible night in 2014, the students were killed – their bodies burnt and dumped in mass graves – by local police officers working for the drug gang Guerreros Unidos. But according to the Mexican journalist Anabel Hernández, the government investigation was plagued with false forensic evidence, confessions elicited through torture and shifting versions of events. Hernández’s book, A Massacre in Mexico, which was recently translated into English, posits that the federal authorities went out of their way to cover up the role of the Mexican army in the disappearances. She suggests that the motive for the attack was $2m of heroin hidden in the buses that the students unwittingly seized and that Guerreros Unidos directed the army to return. One theory is that the army stopped the buses, took back the drugs and wanted to get rid of the witnesses. Whether all the students were killed is still unknown.

Antonio Tizapa hands out signs drawing attention to the 43 at an event in Brooklyn. Photograph: Raul Vilchis

Tizapa believes his son could still be alive and speaks about him in the present tense. Before school Jorge had worked as a bus driver to support his girlfriend and his new baby. It was his wish to go to the Ayotzinapa school and become a teacher, but he struggled to pass the difficult entrance exam and was thrilled when he finally got in. In the months before his death, his first few months in school, Jorge excitedly told his father about his travels as part of the curriculum to some of Mexico’s poorest and most remote regions. And Tizapa would tell his son about how he had started to run.

Now it is the memory of his son that motivates him. During the most difficult points of his long distance runs is when he thinks of Jorge the most. “I think about when he was a child and when he was bigger, his laughter, his words,” Tizapa said, his voice cracking.

“Deep into these races you remember everything.”

Tizapa lives in New York in the same building as his brother but his wife and two other children – a 21-year-old son and 24-year-old daughter – are still in Mexico. His younger son is currently a student at Ayotzinapa.

Tizapa has become an activist. He seeks out every opportunity to speak about his son and demand that the government do more to solve the crime. Last year he confronted Andrés Manuel López Obrador, Mexico’s president-elect, on the campaign trail in New York, pressing him to explain his links with local politicians in Guerrero implicated in the disappearances. López Obrador brushed off his criticisms, according to video of the encounter.

But Tizapa’s quieter moments are spent doing stretching exercises in the park and jogging in the evening alone.

“I come here and I am able to get rid of some of the bad feelings I am carrying. It is a way of being aware,” Tizapa said. “Of course it is painful to train and run in a marathon and I have had moments where, emotionally, I have just wanted to fall down and give up.”

But he runs on.

“At mile 20 that pain in your legs is nothing compared to everything else I am going through,” Tizapa said. “Because after running a marathon you know that in a day or two the pain will be gone. Not knowing anything about where your child is, that is permanent pain.”

Tizapa says he will not stop running until he finds his son.

Considering the slow pace of justice in Mexico, he may be running for the rest of his life.