Two days before the Berlin killings resurrected the darkest period in his life, Conrad Leslie says an unexpected message from a stranger on Facebook gave him and his wife a comfort unlike anything they had experienced since their son, a UC Berkeley student, died in the Nice, France, terror attack in July.

The message, Leslie said, put to rest a recurring nightmare the couple had: a vision of their son’s body on the promenade, alone for hours. In fact, a woman and her daughter had surrounded Nicholas Leslie’s body with candles and prayed for him throughout the night, Leslie learned.

“Instead of running in fear, they asked the police if they could stay,” said Leslie of Del Mar (San Diego County). “It was a cold night, and they stayed with his body. It was like a Christmas story for us — a miracle almost.”

The Facebook message came from a French woman whose mother vacations in Nice every year. She said her mother ran into two women — also a mother and daughter — who told the story of finding a young man’s body along the promenade and staying with him until first responders came.

The woman who learned the story read an article in Le Monde that quoted Leslie as saying no one was with his son. But based on the description of the young man’s body and the location of where he died as reported in the newspaper, she connected the dots and told her daughter, who speaks English, to track Leslie down at once.

After they connected on Facebook, Leslie says he is now trying to track down the mother and daughter who stayed with his son’s body.

“I just know they exist, and we’re going to find them,” he said.

Nicholas Leslie, 20, was in France for a study-abroad program through UC Berkeley, where he studied business and environmental sciences. In the summer program, students were encouraged to develop their startup ideas. His was to create a way to measure lactic acid generation in athletes in real time.

On the night of July 14, Nicholas Leslie and his friends were strolling down the Promenade des Anglais along the Mediterranean. The group had planned to watch the Bastille Day fireworks and get to bed early for a trip the next morning to Barcelona, Spain.

Leslie said his son, with a long striding gait, was about 10 feet in front of his friends, who dived down to the beach when a man driving a cargo truck intentionally ran over dozens of people, killing 86.

“One friend swore he saw Nick run away. Later we found out it was probably PTSD,” Leslie said. “But that helped us get the strength to go to Nice to look for him.”

The uncertainty and pain were recalled in vivid detail Monday when a similar truck attack in a bustling Christmas market in central Berlin killed 12 people and injured dozens. A huge manhunt continued Thursday as German officials worked to track down the attacker.

“It just brings back the moment for us as parents. We felt the desperation, the pain,” Leslie said. “It’s a new world we live in, and it’s terrible.”

Leslie, an engineer and mostly lapsed Catholic, said the writings on death by the 18th century scientist Emanuel Swedenborg helped him cope. Although he’s bothered when people tell him that his son’s death was God’s will or that Nicholas Leslie is in a “better place,” Leslie said he takes comfort in Swedenborg’s idea that two angels carried his son to heaven, embodied by the mother and daughter who prayed over him.

Leslie said that he hasn’t forgiven the attacker — or learned his last name — but that he doesn’t hate the man, who was killed by police in Nice. He also doesn’t want parents to shield their children from visiting other cultures, so he and his wife created a scholarship to help UC Berkeley students study abroad.

“This is an important time to travel abroad and share ideas with other young people,” he said. “There are so many more good people than bad people.”

Kimberly Veklerov is a San Francisco Chronicle staff writer. Email: kveklerov@sfchronicle.com Twitter: @kveklerov