A Minnesota couple charged with abusing their 4-month-old son, who was born conjoined to his brother, were not immediately arrested because they didn't pose a threat to the public and their children had already been removed.

Olmsted County Attorney Mark A. Ostrem explained on Thursday his reasons for not having Valerie J. James, 19, and Robert L. Heck III, 27, arrested on Wednesday, when his office filed criminal charges against them.

He also said they weren't a flight risk. The boys are in supervised care away from their parents.

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The criminal complaint in the case charges James and Heck with on count each of felony first-degree assault and aiding and abetting the alleged crime. Their first court date in the criminal case was set for April 12.

"It's a particularly sad case," Ostrem said, adding, "Because of the nature of the injuries and the nature of the victim, we take it very seriously."

Heck has denied the allegations, but said he would not comment at length until after he had spoken with his attorney.

James gave birth to the boys — named Jacob and Jordan — in November. Joined face-to-face at the abdomen, they immediately underwent successful separation surgery at the Mayo Clinic in Rochester.

Jacob had remained in the hospital after Jordan was released Jan. 3 to his parents, who had been staying at a Ronald McDonald House in Rochester. On Jan. 11, James and Heck brought Jordan back to Mayo Eugenio Litta Children's Hospital because of swollen legs.

Doctors found eight fractures of the boy's legs and 16 rib fractures. The complaint said some injuries appeared to have begun healing, while others were newer.

Doctors did a test to see if the boy had a genetic disorder that could cause his bones to be more susceptible to breaking, but the test came back negative, according to the criminal complaint.

Ostrem said Thursday that he waited for the results of the test, among other things, before deciding to file assault charges.

The criminal complaint also said a former girlfriend had obtained an order of protection against Heck in 2004 after she accused him of hurting her three children.

While criminal case is in Olmsted County, James and Heck are also fighting an attempt by Blue Earth County to severe their parental rights to the twins and their older daughter.

James' attorney in the county children in need of protective services, or CHiPS, case said her client denied abusing her children.

"(James) certainly has denied any of the allegations in the CHiPS case," said attorney Carrie Marsh. "There hasn't been any history (of abuse) with her whatsoever."

Heck's attorney in the parental rights case, Mark Betters, said there was no evidence that the second boy or the older sister had been hurt

He said he had not seen the results of the test for brittle bone disease on the injured child.