Story highlights Boy, 4, is found dead in a luxury apartment building in New York City

He had cuts, burns and visible bruising, authorities say

His baby sitter, 28, has been arrested, but she has not been charged, police say

A baby sitter was arrested after a 4-year-old boy was discovered dead on Wednesday in a luxury New York City apartment building, according to the New York Police Department.

The child was found unconscious and unresponsive with visible bruising, cuts to his arms and legs and burns to his body, said Detective Markowski. (NYPD policy is not to give out the full names of its detectives).

He was pronounced dead at Roosevelt Hospital in New York.

In a new conference, NYPD Commissioner Bill Bratton said the boy suffered "horrific injuries sustained over period of days."

The 28-year-old baby sitter has not been charged, according to NYPD Detective Ort. She did not confess to killing the boy, but police said she made statements to them that she abused the child.

Julie Bolcer, a spokeswoman for the Office of Chief Medical Examiner, said the office is looking into the cause and manner of the death.