The Bronx apartment where a 21-year-old woman was viciously tortured and left to die was so filthy that even the building superintendent refused to go inside, neighbors told The Post.

Princesa Encarnacio-Soto’s blood was still splattered on the mattress inside the Fordham Heights apartment where police said she suffered nightmarish abuse before her body was found dumped in a nearby stairwell on Tuesday.

“We believe that she was tied down to the bed based on the blood that was on the bed,” NYPD Chief of Detectives Joseph Kenny said.

“The wounds themselves were inflicted for pain, not to kill,” he said. “There were slice marks on her thighs, slice marks on her upper arms, and shoulders. The ones on her legs were fresh. The ones on her upper torso seemed like they were in the process of healing.” 

The chief said cops are now looking for Robert Strother, 27, the dead woman’s boyfriend, who is sought as a person of interest.

Meanwhile, police have detained Strother’s mother, 54-year-old Naida Jorge, for questioning. She has not been charged.

“We are still holding her,” Kenny said. “She did attempt to clean up the crime scene. There was no blood on [Encarnacion-Soto] while she lay on the staircase, and she was in fresh clothing that had no blood on her. 

“So, [there is] a possibility that someone cleaned the body, put fresh clothes on her, and threw it on her by the staircase.”

A photo obtained by The Post from inside the Grand Concourse apartment shows layers of grime and debris inside the home, where police said Encarnacion-Soto was tied up, stabbed from head to toe, and possibly beaten with a hammer before she was dumped in a nearby stairwell on Tuesday, police and sources said.

“Five months ago I fixed her apartment,” the superintendent, who asked not to be identified, said this week. “I worked for two weeks in the apartment. I fixed the kitchen — everything.

“Everything was dirty, the whole house,” the man added. “The bedrooms where they sleep was dirty – they smoke. I didn’t want to go in there.”

According to law enforcement sources, the young victim was viciously tortured amid the squalor.

Cops responding to a 911 call Tuesday morning found the victim barely alive in the stairwell, and followed a trail of blood that led into the apartment where she was believed to have been tortured.

She was rushed to a BronxCare Health System facility, where she was pronounced dead.

Kenny said she had moved into the Bronx apartment as recently as July 11, but was originally from Rhode Island. 

Strother, her 27-year-old boyfriend, was spotted leaving the Bronx building on security cameras, sources said.

Neighbors said Jorge had lived there for more than 20 years — and said problems inside the apartment were known to many in the building.

Sources said the older woman became uncooperative when questioned by police.

In a cruel twist, Kenny said police are also looking at a 911 call in May during which Strother allegedly claimed Encarnacion-Soto was raped by a group of men on the roof of the building.

But cops determined the alleged rapist was in prison at the time, and believe the call was an attempt to hide the ongoing abuse that Encarnacion-Soto had been suffering in the apartment.

“The mom and son fabricated the rape story because our victim here was injured to the point she needed medical attention,” he said.

It is not clear why the injuries did not trigger an investigation.