(CNN) Adam Smith said he's known about the white cardboard box in his mother's freezer his entire life.

He assumed it held a wedding cake topper or another memento that his mother was holding onto and left it at that, he told CNN.

Now that he knows what was really in the box, he said he is struggling to comprehend how she kept it to herself all these years.

Smith, 37, called police early Sunday morning and reported finding a dead infant in the box.

Adam Smith said this photo shows the box inside a freezer where he found a dead infant.

St. Louis police said they are investigating the discovery as a "suspicious death" pending an autopsy and asked the public for tips. The St. Louis Medical Examiner's office said the investigation is ongoing and would not share details.

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