Cops investigating the mysterious death Wednesday of a 4-year-old boy at a Midtown apartment said the child had third-degree burns, bruises and cigarette marks all over his body.

Miles Wade was rushed from the Ritz Plaza luxury apartment building on West 48th Street to Roosevelt Hospital shortly before 11 a.m. after his baby sitter said the child was unresponsive.

The boy was pronounced dead at the hospital.

The baby sitter told cops that the boy fell from a bathroom counter into an empty bathtub after she stepped out for a moment.

But a source said a preliminary examination revealed cigarette burns all over the boy’s body as well as bruises, cuts and scrapes.

Some of the wounds were described as “defensive.”

Paramedics who responded found the boy curled in the tub with foam and liquid coming out of his mouth.

The medical examiner will perform an autopsy.

“I think it’s really sad,” said neighbor Axel Morgan, 28. “Usually nothing happens in this building. It’s a really calm neighborhood even though we’re next to Broadway.”

Sources said the boy’s father is in a New Jersey prison. There was no immediate information about the boy’s mother.

The sitter has been helping to take care of the child since December, they said.