A 17-year-old boy was being held in the Hennepin County juvenile detention center Thursday night in connection with an investigation into the death of his girlfriend's 2-year-old daughter.

Minneapolis police released few details about the case ahead of the official medical examiner's report.

The child's biological father, Max O'Neill, said in an interview late Thursday that his daughter, Sophia O'Neill, died of internal bleeding. O'Neill said doctors at Hennepin County Medical Center told him the toddler had suffered a ruptured kidney and pancreas. The father said he had seen injuries on his child previously that he suspected were the result of abuse and reported them to authorities.

However, the suspect's grandmother described the teenager as a loving caretaker, who frequently took care of the girl while the mother was away.

Just before noon Wednesday, officers responded to an emergency call about a child not breathing in the 3800 block of 30th Avenue S., a residential area southwest of Hiawatha Avenue and E. 38th Street.

The toddler was taken by ambulance to HCMC and later died.

Police would say only that "a juvenile suspect" was arrested in connection with the child's death, declining to release any other details.

Interviewed Thursday night at the home, the suspect's grandmother, who asked that her name not be published, said she was grieving for Sophia.

She said the child was the daughter of her grandson's 20-year-old girlfriend.

She said the pair had been dating for about a year and they had both been staying with her temporarily.

She said her grandson often took the child to a nearby park, and that he treated her with affection.

"He loved her," the grandmother said. "I never saw any signs of abuse or neglect."

But Max O'Neill, 22, who had shared custody with Sophia's mother, said he'd seen bruises on the young girl before and reported his suspicions to Child Protective Services.

He maintained that officials there didn't investigate. O'Neill said he last saw his daughter at the end of April.

Sophia was described by the teen's grandmother as a bright child who was in the process of learning her numbers.

The grandmother said the toddler had a high fever and was vomiting Tuesday, but seemed better Wednesday after being given pain relief medicine.

The grandmother went to work Wednesday morning and Sophia's mother went to her own doctor's appointment, while the 17-year-old boy stayed home with the baby.

The grandmother and a next-door neighbor, who asked that his name not be published, described in an interview the moments just before paramedics were summoned.

The neighbor said the 17-year-old frantically banged on his door while cradling the girl.

Her skin was turning blue and she was not breathing, he said. The neighbor said he called 911 and began light chest compressions on the child.

Police and paramedics arrived almost immediately, he said.

The grandmother said their house does not have a land line phone and that the 17-year-old did not have a cellphone, so he had to run to the neighbor's for help in calling for paramedics.

Several other neighbors expressed shock Thursday that a child had died on their block.

They also said that the home has drawn a fair amount of police attention in the past, with young people often gathering there.

The 17-year-old's grandmother said she visited him at the detention center Thursday morning and told him "to stay strong."

She said she doesn't believe the boy harmed the child, and plans to support him through the investigation.

The county attorney's office has until noon Friday to charge the suspect or to ask for an extension.

Staff writers David Chanen and Paul Walsh contributed to this report.

Liz Sawyer • 612-673-4648