A 5-year-old boy who was stabbed in the neck and found facedown in the bathtub of his West Philadelphia home on Monday night might have been killed by his mother, police said.

The boy’s father found the child submerged in the tub when he returned to his apartment on the 4800 block of Pine Street about 10 p.m., according to police. The father found his wife, 40, in the bedroom cutting her wrist, they said.

The boy was taken by medics to Children’s Hospital of Philadelphia, where he was pronounced dead at 11:03 p.m.

His mother was taken to Penn Presbyterian Medical Center, where she was in stable condition. Police on Tuesday said she is in custody, but did not provide any further details.

Police Chief Inspector Scott Small told reporters outside the family’s home shortly after the episode that the mother may have mental-health issues.

Police did not release the names of the child, the mother or the father. The case remains under investigation.

On Tuesday, five blue balloons, which were initially placed on the steps outside the apartment building, were later tied to a black metal railing by the steps. As they blew in the wind, a taped note below them read: “Remembering you at this time with deepest sympathy.”

One man who lives in the brick building in the Garden Court section of West Philly said he doesn’t know the family well, but the mother seemed “always friendly."

Maria Jones, 63, who lives several blocks away, stopped her car on the block after spotting the balloons. “It’s just heartbreaking that somebody could do this to a baby,” she said, adding that she has a 5-year-old granddaughter. “Nobody will probably ever know what happened in that unit,” she said as she stood across the street from the apartment building.