The old sidewalk sweeper spotted the swollen cardboard box Wednesday morning on the curb in front of the Rainbow Day Care Center in North Philadelphia.

Homer Keller, 77, who works for a church that owns properties on the battered street, didn't think anything of it, and, in the mild winter weather, raked two lots of tall grass, and swept outside a shuttered car wash, padlocked bar, and broken-windowed detail shop.

An hour later, he made his way back to the box. It was about a foot high and a foot wide, with some dirty clothing showing. He knocked on the day-care center door. Not ours, the staff told him.

He hefted the box, feeling its weight, and carried it to his trash pile.

First came whimpering.

"I thought it was cats," Keller said Wednesday night on the porch of his Germantown home. "With the lots on that street, there's cats always getting in the trash."

Then he heard a baby crying.

"I thought I must be mesmerizing," said the South Carolina native. "I pulled the covers back, and here was this baby."

He saw a girl, still blood-streaked, her umbilical cord still attached. There was no note. He dialed 911.

"It scared me half to death," said Keller, a father of seven. "How could someone do such a thing?"

Paramedics cut the umbilical cord. Doctors at St. Christopher's Hospital for Children reported the full-term baby in good condition with no signs of trauma. From her body temperature, they believe she spent almost every minute of her first five hours in the box at 26th and Huntingdon Streets.

Investigators canvassed the neighborhood and reviewed security footage but developed no leads to the mother's identity, said Lt. Anthony McFadden of the Special Victims Unit. All emergency rooms in the area were checked for patients who recently gave birth. The Crime Scene Unit was examining the box for fingerprints.

At a news conference Wednesday night, McFadden displayed photos of the baby's swaddlings: an adult woman's brown sweater, a red-and-white houndstooth blanket, a black bathrobe with pink polka dots, and a peach-colored bath towel.

"Right now, we are most concerned about the health and well-being of whoever gave birth to this child," he said.

The District Attorney's Office will decide on criminal charges if the mother is found, McFadden said. Anyone with information is asked to call the Special Victims Unit at 215-685-3251.

Pennsylvania has a baby "safe haven" law that allows parents to leave unharmed babies up to 28 days old at any hospital.

"She took the time to wrap this child up like this, and she could have taken her to the closest firehouse, police station, or hospital with no questions asked," McFadden said. "Just to leave her on the sidewalk - it's obviously very frustrating."

Keller, a Korean War veteran and retired commercial truck driver, returned home Wednesday night after talking with investigators.

He said they showed him a picture of the girl wrapped in a clean blanket at the hospital.

"She was really cute," Keller said, smiling. "I'm really glad I found her. I'm glad anyone found her, because them trash trucks that come by, they wouldn't have checked, they would have just took the box and thrown it in the hamper."

Contact staff writer Mike Newall at 215-854-2759, mnewall@phillynews.com, or @MikeNewall on Twitter.