A South St. Paul man, who admitted to police that he has a short temper, squeezed a baby boy until the 6-week-old infant's ribs broke, according to criminal charges filed in Washington County.

Mark James Nelson, 32, has been charged with felony first-degree assault with great bodily harm in connection with the boy's injuries, which occurred Aug. 19 in a Forest Lake residence.

"It's just awful," County Attorney Pete Orput said Tuesday. "They don't get more vulnerable than a 6-week-old baby. It's just tragic."

Orput said the baby remained in the Hennepin County Medical Center intensive care unit.

According to the complaint, doctors at the hospital found 37 breaks in 22 of the child's ribs. Only two ribs were undisturbed. The boy also was bleeding from internal injuries.

The boy's mother told police that when she lifted him "he felt crunchy," referring to his ribcage. She said that the boy was not "smiley and go lucky" as usual and that he "looked sad."

She said she had dated Nelson, who is not the boy's father, since four months into her pregnancy. He had been present at the boy's birth.

Nelson, in the complaint, said he had squeezed the baby after an argument with the boy's mother. He said he stopped when he heard something crack.

The day after the alleged assault, Nelson told investigators, the baby twice stopped breathing. Nelson said he revived the baby but didn't seek medical attention.

Nelson, who said he loved the boy, remains in the Washington County jail on $200,000 bail. His next court appearance is Sept. 8.