A newborn baby was killed when his mother tossed him from a fourth-floor window of their New York City apartment building and he landed on courtyard pavement 40 feet (12 meters) below, a prosecutor said Saturday in announcing her arrest on a murder charge.

Rashida Chowdhury, 21, was awaiting arraignment a day after 3-week-old Rizwan Ahmad was found dead in a onesie on the barren courtyard of the building in the borough of Queens.

"This is a truly horrific and disturbing case," Queens chief prosecutor Richard Brown said. "The alleged actions of the defendant are totally incomprehensible."

Medical examiners ruled the baby's death a homicide, saying he died of blunt impact injuries and had suffered skull fractures and lacerations to his brain, liver and spleen.

Neighbor Mazol Ilyayeva told reporters she heard something fall around 4 a.m. Friday and then saw the child's lifeless, bloody body in the courtyard.

"I couldn't move. I couldn't scream," she said. "I couldn't even look. ... It's horrible."

She said the parents had seemed stricken as they stood in the courtyard afterward.

"The mom was holding the husband," Ilyayeva said. "Their faces were blank."

It was unclear if Chowdhury had a lawyer who could comment on the charges; no telephone number could immediately be found for her Queens apartment.