Cops on Saturday were grilling a male cousin of a 35-year-old Manhattan mom discovered dead in a storage container, police sources said.

The victim, Nisaa Walcott, of East Harlem, was found Friday afternoon inside a plastic container that was left across the street from a storage unit at 950 University Ave. in the Highbridge section, sources said. The woman was reported missing earlier this week, the sources said.

Cops made the grisly discovery just before 2 p.m., four blocks away from Yankee Stadium, according to the NYPD.

Police say they have video of Walcott going into her East Harlem apartment building as well as the suspect — but only the suspect leaving, the sources said.

The police sources also said video reviewed by detectives show the cousin at Walcott’s building, removing a plastic container early Friday.

Cops believe the suspect texted concerned family members from Walcott’s cell phone to throw them off, but the family grew suspicious because the answers were inconsistent with previous conversations — including a text that said she was going on a business trip and the suspect would watch her son. 

Investigators were awaiting autopsy results for Walcott Saturday, and had been questioning her cousin at the 23rd Precinct in East Harlem, police sources said.

Christopher Sanchez, a neighbor, said Walcott was a doting mother to her 14-year-old son.

“They were close,” he said. “When it came to her son, that was her top priority. That’s a sad thing,” he said.

Another neighbor and friend called Walcott a “beautiful woman.”

“She gave a big smile to everyone. She cared for her son. She was a great woman, a hard worker. She would come from work and go home,” he said.

He said Walcott let her cousin stay with her.

“How do you do that to your cousin? She gave you shelter, she gave you a place to live. She actually extended her hand and this is how you repay her?” he said.