NEW YORK (NBC) -- Police say a New York City high school teacher was on the phone with a 911 operator to report her son was having a seizure when the 19-year-old attacked her and beat her to death in their Hell's Kitchen apartment.

A police source said the assault was in progress as Karyn Kay, 63, told a dispatcher in the disturbing call that her son was having "seizures."

One neighbor reported hearing someone scream, "I'm sorry, Mommy."

When officers responded, they found Kay lying bloodied and unconscious on the ground inside the apartment.

Authorities say she suffered broken ribs, a fractured skull and internal bleeding.

An autopsy was scheduled for Wednesday.

Kay's son Henry Wachtel was arrested and charged with murder.

It wasn't immediately clear if he had an attorney.

He was taken to Bellevue for psychiatric evaluation and was awaiting arraignment Wednesday.

He has no history of violence, but had a history of seizures.

There was no indication he used a weapon, police said, and investigators were trying to determine what happened.

Police could offer no motive in the slaying.

Kay taught creative writing at Manhattan's famed LaGuardia High School and at Pratt Institute in Brooklyn.

She wrote and produced the feature film "Call Me."

Neighbor Debra Alberts said she was shocked when she learned what happened on the 10th floor of her apartment building.

"The doorman shared with me they had just taken the son out in handcuffs because he had beaten his mother up very, very badly," said Alberts.

"She's just plain, normal, sweet lady," said another neighbor, Hasiba Ali. "I can't believe that. I'm in shock."

Neighbors said they saw frequent arguments between Kay and her son but dismissed them as typical teenage behavior.

"You see young kids mouthing off to their parents, and you would see it on occasion in the elevator from time to time, but it was nothing unusual," said Alberts.