Special Victims Unit officers questioned Brown and her boyfriend later Tuesday and released them, pending the results of a medical examiner's report, police sources said.
The report found that the girl died of laceration of the liver from blunt-force trauma, and the death was ruled a homicide, a police source said. The Homicide Unit then took over the case.
Brown told investigators that when she gave Seanita a bath late Monday, the girl looked "out of it," the source said. She took Seanita to the hospital. Doctors noticed the bruises on the girl's body and notified authorities. Brown and her boyfriend could not explain the bruises, the source said.
The city's child-welfare agency was involved with the family and had investigated an allegation of abuse against Seanita weeks before her death, according to a relative of the girl's who asked not to be identified.
In June, Seanita's day-care center notified the Department of Human Services that Seanita was walking with a limp and had bruises on her leg.
A DHS social worker took Seanita to Children's, where she was examined by doctors. The doctors found that Seanita's injuries were consistent with the mother's story that Seanita had accidentally fallen, the relative said.
As part of the investigation, the social worker visited the 56th Street home about two weeks before Seanita died, the relative said.
Alicia Taylor, a DHS spokeswoman, said confidentiality restrictions kept her from commenting on the case.
As with every child fatality, DHS is conducting an internal investigation and assisting police, Taylor has said.
Josephita Brown is scheduled to be arraigned Friday morning.
Contact Mike Newall at 215-854-3919 or firstname.lastname@example.org, or follow @MikeNewall on Twitter.