The 5-year-old twins who were found lifeless in their mother’s bed in the family’s Bronx apartment last December were smothered to death, according to the city’s medical examiner.

The deaths of the children, a boy and a girl, have been ruled homicides, said Julie Bolcer, a spokeswoman for the medical examiner’s office. The cause and manner of their deaths were determined on Wednesday evening.

So far, no arrests have been made, a police spokeswoman said on Thursday.

The twins’ deaths had puzzled the authorities for nearly three months. The police saw no visible signs of trauma on their bodies, nor did officers find any weapons or narcotics in the family’s sixth-floor apartment on East 175th Street near Monroe Avenue in the Mount Hope neighborhood.

Initial autopsies were performed, but they required further study, according to Ms. Bolcer. The medical examiner has not specified what those studies were, though certain tests, including toxicology, neuropathology and histology — a close examination of tissue — often take longer to return results.

On Dec. 18, the mother, whom the police have not named, told officers she had slept in the same bed as her children the previous night. She said the last time she saw the children alive was around 5 a.m.

When she woke at around 11:20 a.m., she said, she found them stiff and cold. Emergency workers arrived soon after and performed lifesaving measures. The children were pronounced dead 10 minutes later. The boy and girl were discovered foaming at the mouth, Joseph Kenny, the chief of detectives for the Police Department, told reporters later that week.