A college student is accused of killing a 6-week-old baby and abusing his twin brother while visiting the children's parents in Pennsylvania.

Nicole Virzi, 29, a doctoral candidate at San Diego State University and UC San Diego, was in town for the weekend. She was babysitting Leon Katz when he suffered multiple skull fractures under her care at the parents' Pittsburgh apartment.

She also stands accused of abusing Leon's twin brother after several injuries were found on the boy's face, belly, and genitals earlier that same day.

The 29-year-old was arrested on suspicion of murder, child endangerment. and aggravated assault. She pleaded not guilty and remains in custody at Allegheny County Jail.

David Shrager, Virzi's defense lawyer, says Leon fell out of a bouncing high chair while the defendant was out of the room, denying any wrongdoing on her behalf.

"We are at the beginning of our investigation, but it's important to note that my client has no criminal history whatsoever," Shrager told the San Diego Union-Tribune. "She is a Ph.D. student and was a longtime friend of the family." 

The complaint states Virzi had been visiting with the family when she found blood in the diapers of Leon's brother Ari. 

When questioned by officers, the parents said that Virzi had been alone with the baby just before she told them about the injury.

While the parents rushed to a children's hospital with the injured child, Virzi stayed behind at the home to watch Leon.

She would later tell the police that she had the baby in a bouncy chair while the parents were gone and occasionally changed, burped and fed him throughout the evening. 

However, around 11 p.m., she claims she went to the kitchen to get a bottle for the child but forgot to strap him into the chair.

That's when she heard the baby scream from the other room and found Leon on the ground with a "large bump on his head."

Virzi called 911 to report the fall and told dispatchers the baby was conscious but struggling to breathe.

Leon was transported to a children's hospital by paramedics, where he was pronounced dead.

A CT scan revealed he had suffered a skull fracture along with multiple brain bleeds.

A doctor who examined the twins told police that the injuries they suffered were consistent with child abuse and "not accidental," WTAE-TV reported.

Investigators said Virzi could not provide a "plausible explanation" for the injuries. 

In a GoFundMe created for the Katz family, Leon is described as a beautiful and healthy six-week-old baby boy who was killed under horrific circumstances.

Funds raised will cover funeral expenses as well as medical bills resulting from injuries both boys sustained while under the care of a "trusted family friend."

The page described Leon as a happy, smiley, goofy baby boy.