(CNN) -- It was a medical mystery of a different kind, one that warmed the hearts of those who needed it most.
And now it's been solved.
In the snow that piled on the parking deck rooftop of Chicago's Rush University Medical Center, someone wrote in large, clean letters: "HI MOM GOD BLESS U" (with a smiley face for the 'O," for good measure.)
The message has brightened the bleak hours of many, according to Angela Washek, a nurse in the hospital's intensive care unit.
It appeared early Sunday morning, after Washek saw a group of men running around on top of the parking garage.
"I didn't know what they were doing," Washek said.
She took a closer look when a patient's daughter stood at the window of room 1155, and began to cry. Her mother had yet to wake up from surgery.
That's when Washek realized it was more than a group of men playing around on top of a parking garage. What she saw was a brother, a father and an uncle writing an uplifting message in the snow.
"My patient's daughter wished she could have done something like that for her mother," Washek said.
The incident was first reported by CNN affiliate WGN.
The hospital posted Washek's photo of the rooftop gesture on the Rush University Medical Center Facebook page. More than a thousand people have liked the photo, and within the 50 comments posted, there was an answer.
Hannah Hart explained the message was intended for her mother, Sharon, a leukemia patient at the hospital. Sharon's family wanted to raise her spirits before visiting hours. Initially, "Hi Mom!" was written in the snow. The trio later tried to form "GOD BLESS U ALL" to make the message meaningful to more patients, but they ran out of parking space.
The message could be seen from 70 to 80 hospital rooms, across four floors covering the oncology, surgical and intensive care units.
Deb Song, a hospital spokeswoman, said the simple message inspired patients and doctors alike.
"They wanted to make everyone feel good, and to let everyone else at the hospital know they were thinking of them," Song said.
The gesture is now covered by parked cars, but the sentiment of the message lingers.
"It reminds us," Washek said, "to fight for their families, as well as our patients."