Zanis began making those crosses in 1996, the same year his father-in-law was shot and killed.
Then he was contacted by the mother of Nico Contreras, a 6-year-old who had been shot and killed in his hometown of Aurora, Illinois. She asked him to build a cross in Nico's honor.
In 1999, he received a call from the family of one of the victims of the Columbine school shooting. He traveled to Colorado to deliver their crosses and began traveling more and more to the scenes of mass shootings.
Aurora man who built more than 27K crosses to honor gun violence victims retires
He traveled to Newtown, Conn., in the wake of the Sandy Hook shooting, to Las Vegas after the massacre at the Harvest Music Festival, and to Orlando to commemorate the victims of the Pulse Nightclub shooting, even though he got pushback from his church.
Not even a year after the Las Vegas shooting, Zanis traveled to Santa Fe, Texas, where 10 students and teachers died in a high school shooting.
10 white crosses honors victims of Santa Fe high School shooting
He did not just create crosses; he built Stars of David and other symbols to honor the victims of senseless violence.
Last week, a tribute was held for Zanis as supporters held a drive-by parade to show their appreciation.
Aurora car parade pays tribute to man who built more than 27K crosses for gun violence victims
Zanis retired from making crosses late last year, without mentioning his illness.
He asked the group Lutheran Church Charities to take over, and they're discovering just how large an impact Zanis made.
"Who knows the number of lives he's touched. Just an incredible man," Tim Hetzner, CEO of Lutheran Church Charities, said.
Aurora Mayor Richard Irvin issued a statement saying, "Mr. Greg Zanis was a giant among men. He was a man of action who simply wanted to honor the lives of others. In return, his life was one of honor and one that was celebrated throughout our nation and world. Heeding to the scripture 'pick up your cross and follow me,' Mr. Greg Zanis did just that. He picked up the crosses he made and followed his mission in the noblest of ways. His legacy shall forever be remembered in his hometown of Aurora and around the globe."