At the scene of some of the nation's worst tragedies — mass shootings and other acts of terror, natural disasters — there has been a constant: handmade white crosses neatly arranged and bearing the names of every victim killed.

It was a memorial project that began in 1996 and was conceived by an Illinois carpenter named Greg Zanis, who told NBC News on Thursday that "it evolved quickly into something I certainly didn't expect to have that great of a need."

But in recent years, the attacks seemed to become more frequent and more gruesome: Twenty-six children and adults killed in the 2012 Sandy Hook Elementary School shooting in Newtown, Connecticut; nearly 50 people killed in the 2016 Pulse nightclub shooting in Orlando, Florida; and more than 50 in the Las Vegas concert shooting the following year.

Rev. Arthur Thomas, right, of Mt. Nebo Baptist Church, right, is embraced by Greg Zanis who built a cross for each victim of the tornado and placed them as a makeshift memorial in Beauregard, Ala., on March 6, 2019. David Goldman / AP

Now, Zanis, 69, has decided it's time to retire from the self-appointed role he's held for more than two decades building the crosses (or stars of David or crescent moons depending on the victims' religious affiliation) and transporting them to each scene of violence and loss.

"I just feel like it was a calling only I could have filled," Zanis said, estimating he has built about 27,000 crosses since launching Zanis Crosses for Losses, which has been funded out of his own pocket and through donations.

Ahead of his decision to step back from his work in November, this year was particularly exhausting, he said.

In February, Zanis' hometown of Aurora, Illinois, was hit by gun violence when a former warehouse employee killed five people and injured six police officers. Zanis built crosses for all the lives lost.

Then, in August, he drove from the Chicago area to El Paso, Texas, where 22 people were fatally shot at a Walmart. After arriving and setting up the crosses in their honor, he was startled to learn of another mass shooting the next day in Dayton, Ohio, where nine people were killed outside a bar. He immediately drove his pickup more than 1,500 miles to deliver crosses to that scene.

He spoke with NBC's Lester Holt from El Paso about what kept him going: the approval from grieving loved ones.

"This is all they have left," Zanis said at the time, "and I know that and I know it means the world to them."

Carpenter Greg Zanis sets up wooden crosses in memory of the victims of the Camp fire in Paradise, Calif., on Dec. 19, 2018. Barbara Munker / picture alliance via Getty Image

Zanis said his mission began as a tribute to his own father-in-law who was murdered in 1996.

Zanis first brought the crosses after the 1999 massacre at Columbine High School in Columbine, Colorado, and he would go on to other places touched by death or disaster, such as Parkland, Florida; Boston; and Paradise, California.

In the years he has been doing this, he said, his phone rings almost immediately after a mass shooting happens; people look up his name because sometimes they don't know where else to turn.

In many cases, he added, he actually knows the victims' names before officials release them because families and friends reach out to him.

The work, however, has taken an emotional toll on him, and while he tried to stay strong wherever he went, there were moments when he would break down in tears. He hopes his past efforts have given others a sense that they are not alone.

"I saw it as something that makes a difference, and touches families, the city, state and nation," he said.