The boy was running around with flyers, asking anyone who would listen whether they knew about his mother. She disappeared in 2012 from a job interview.
Tadeo misses her. He told Lopez that the people who took his mother should give her back because he needs her more than they do.
"If they can give me an address, I'll pick her up," Lopez recalled the boy saying.
Like so many families, Tadeo's family is in limbo.
Since 2007, at least 26,000 people have disappeared in Mexico, according to the United Nations. Countless more are grieving the sudden disappearance of a loved one. Lopez's photographs aim to capture the experiences of their loss.
In one image, a young girl and her shadow can be seen against a set of stairs.
Another shows a small living-room altar.
In a third image, a mother sits on her daughter's bed and cries into an oversized, star-shaped pillow.
Lopez's work focuses on the heartbreak of families with missing women and girls. The photographs, which were made in 2014 and 2015, show what is typically a very private pain. They are personal and intimate.
The families feel helpless, Lopez said. Corruption is rampant. Crimes go unpunished.
Police sometimes ask the families for money to pursue their investigations, according to Lopez. Other times, the families dig into the cases themselves.
Tadeo's family, for example, has searched for his mother for years. She was believed to be in Ciudad Juarez at one point -- a border city once nearly synonymous with murder and missing women. The trail has since gone cold.
"I think every family looks for a way forward," Lopez said over email. "They try to focus their energies on other family members and their work. Many look to religion for help."
When a loved one dies, you bury the body and begin to process the pain.
But what happens when families don't know the fate of their mothers or daughters?
"You cannot close the wound and heal," Lopez said. "It is always an open wound."