Shaken-baby syndrome has been estimated to occur in up to two dozen cases per 100,000 infants. In babies ages 2 to 12 weeks, meanwhile, studies suggest that unexpected bleeding due to a vitamin K deficiency occurs 1.4 to 7.2 times per 100,000 births.

That's about 60 to 300 U.S. babies a year.

"If we send all those parents to prison, that's not very good medical practice," Morton said. "What does our legal system allow? Does it strive to put as many people as possible in jail so we don't miss any cases?"

Still, the abnormal result of Lucas' PIVKA blood test wasn't enough for Morton to tell if Alejandro was innocent.

Bone and brain

Like most babies, Lucas was given a dose of vitamin K at birth. Later on, babies rely on vitamin K made by the mother's intestinal bacteria and transferred through breast-feeding.

Yet Lisa told Morton that she had taken potent antibiotics for mastitis, a breast inflammation; he figured this had reduced her - and Lucas' - ability to make vitamin K.

Morton and Rorke-Adams spent hours poring over Lucas' records, piecing together how the baby died. They also enlisted a neuroradiologist to look at the brain scans, and another radiologist and a pathologist to look at the rib fracture.

They found some unusual things. There was some evidence of new bone formation, indicating the break was weeks old. But the growth was abnormal: The broken edges of the fractured rib had not rejoined and were rounded.

Morton suspected that the break had happened during delivery, and that healing had been slow because vitamin K plays a role in bone repair. Broken bones during delivery are unusual, but Lucas was a big baby, and the birth was difficult.

In any event, the fracture did not happen the day Lucas' father was taking care of him, an opinion shared by Geisinger physicians. And if Lucas had been shaken hard enough to die, Morton's team felt, the edges of the rib fracture likely would have caused bruising in the surrounding tissue. This did not happen.

Then there was the brain. This was the specialty of Rorke-Adams - a legend in the field, having taught legions of neuropathologists in Philadelphia since the 1960s.

When Lucas was admitted to Geisinger, an initial CT scan revealed thin subdural hemorrhages - blood just beneath the brain's tough outer layer, or dura. A serious problem, and a possible indication of abuse.

But two days later, Rorke-Adams saw, a second scan showed matters had gotten much worse. There was more subdural bleeding, and new areas of bleeding inside the brain, in the frontal lobes.

Moreover, the bleeding in Lucas' eyes got worse after an initial inspection, and his retinas developed folds that apparently were not present upon admission.

Rorke-Adams has testified across the country in child-abuse cases - for the prosecution. This time, she didn't see evidence of a crime.

In her report, she wrote that Lucas' downward spiral after hospitalization was "not consistent" with what she normally saw in an abused child.

She also noted that the baby's liver and lungs had been unusually heavy. Later analysis found iron deposits in both organs - evidence of some unspecified disease, she and the team decided.

Lucas didn't have the same illness as the Amish girl, Sara Lynn Glick, but his liver, like hers, seemed to have trouble using vitamin K. And he had an inadequate supply of the vitamin to begin with.