It began a week ago with a few flowers placed on a patch of grass. It has grown to a shrine spreading across a park, along fences and onto a hilltop overlooking Columbine High, reports CBS News Correspondent John Blackstone.
Among those still adding to this memorial was Louis Graybill.
"Just some flowers," Graybill said. "Mostly my thoughts and love."
Many have left their thoughts in letters and signs, in poetry and prayers.
At the Colorado Historical Society, Curator Stan Oliner is already making plans to turn the makeshift memorial into a permanent collection.
"Each of the thousands of people that have brought something to that park, the minute they put it there, it became sacred," Oliner said.
The memorial, like the one that grew up spontaneously along the fence in Oklahoma City, like the one outside Kensington Palace for Princess Diana, is a new kind of expression of public grief.
"No longer does the public line up in hour-long lines to sign a condolence book, which is then sent to the family," Oliner said. "We want to personalize that: Here is my public grief as of this day."
Michael Herold came to the memorial to make angels in clay.
"ItÂ's going to rain this afternoon. IÂ'm going to write on the back of this [angel], grief. I hope the rain melts away a lot of the peopleÂ's grief," Herold said.
The permanent memorial may be years away. But what exists now will become part of the history of this tragedy Â— a history written one heartfelt message at a time.