Ph.D. student specializing in depression charged with killing friend’s infant while babysitting twins

Two parents put their twin babies' lives in the hands of a "trusted family friend," who allegedly killed one of the boys and abused the other, according to prosecutors.

Nicole Virzi, a fifth-year Ph.D. student specializing in research about depression's effect on human behavior, babysat 6-week-old twin boys, Leon and Ari Katz, on June 16. 

That's the day Leon died. Virzi said he fell from his bouncer chair after she woke up from a nap and walked away to get a bottle.

But doctors...