A 7-month-old Montgomery County girl who police say was killed by her 14-year-old brother last week died of asphyxiation, according to prosecutors who spoke in court Monday.

The suspect, Jonathan Aguiluc, made his first courtroom appearance Monday, speaking over a video feed from the county jail. Draped in dark green inmate clothes, Aguiluc looked tiny compared with the series of adult suspects appearing before and after him. His attorney, Mary Siegfried, told District Court Judge Karla Smith that he had never been in legal trouble before and that he took care of his infant sister at night while their mother worked.

“He has had no prior contact with the Department of Juvenile Services,” Siegfried said. “He is not a problem in school. He goes to school every day.”

Smith ordered Aguiluc to be transferred to a juvenile detention facility but held without bond.

Police say Aguiluc killed his sister sometime Thursday night or Friday morning inside an apartment in the White Oak section of Silver Spring. Another child, a 3-year-old girl, was also home at the time.

In arrest documents, detectives said the children’s mother left for work at 10:30 p.m. At some point, Aguiluc beat his sister and then covered her mouth and nose with his hands, police said. The detectives also wrote that there were “suspicious injuries” on the infant’s face, chest, back and buttocks. On Monday, prosecutors said the injuries were inflicted by insects.

“They were bug bites that occurred after the child died,” Montgomery State’s Attorney John McCarthy said.

The children’s mother returned from work at 5:30 a.m. Friday. She saw the girl, Larissa Yanes, in a car seat in the living room of their apartment, according to charging documents. She thought Larissa was asleep. A half-hour later, she tried to wake the baby to feed her but found her unresponsive, according to the police. Paramedics were called, and they took the child to Holy Cross Hospital, where she was pronounced dead at 6:54 a.m., according to police.

Detectives interviewed Aguiluc, who said that he beat his sister, which made her cry, according to police. So he placed his hands on the baby’s nose and mouth, restricting her breathing, detectives alleged. At that point, the child stopped crying. Aguiluc carried her in his arms around the apartment, placed her in the car seat and went into a bedroom, police said. Detectives charged Aguiluc with first-degree murder, which by law requires them to charge him as an adult.

Prosecutors are working with detectives to decide whether Aguiluc should ultimately be charged with first- or second-degree murder, indicating that the crime might not have been premeditated. Siegfried, the boy’s attorney, said she would move to transfer the case to the juvenile court system if it doesn’t happen automatically by virtue of a lesser charge.