Cops have arrested the 35-year-old suspect they believe killed a great-grandmother in an East Harlem housing project.

Gregory Velez of The Bronx was charged today with the murder of Julia Hernandez, 73, who lived in the Wagner Houses, police said. He was also slapped with a robbery charge.

Velez had been picked up yesterday around 4 p.m. and spent the night being grilled by detectives, sources said.

Velez, whose rap sheet includes convictions for robbery and drug sales, is the father of Hernandez’s great-grandchild, said neighbor Albert Rosado.

Rosado said Hernandez wouldn’t have opened her door to anyone she didn’t know. “She was careful. She wouldn’t let anyone in her house unless she knew them,” he said.

Hernandez’s daughter found her in her apartment at about 7:40 p.m. Tuesday. Cops said her feet were bound by a scarf and she had a plastic bag over her head.