A 14-year-old Silver Spring boy accused of beating and suffocating his 7-month-old sister has been charged with first-degree murder, Montgomery County police said Saturday.

The suspect, Johnathan Aguiluc, has been charged as an adult, police said. He was being held without bond Saturday in the Montgomery County jail.

Early Friday morning, police and paramedics were called to an apartment on Lockwood Drive in the White Oak section of Silver Spring where a child had been reported as unresponsive. The child was taken to Holy Cross Hospital, where she was pronounced dead at 6:54 a.m. Police identified the girl as Larissa Yanes, and said she lived at the apartment.

Detectives observed injuries on the child they determined could not have been self-inflicted. They learned that the night before the 39-year-old mother of both the teenage boy and infant girl left for work at 10:30 p.m. According to police, the little girl was being watched by her brother.

During an interview with detectives, police said the teenager admitted to beating his sister, which caused her to cry. He also said he covered her mouth and nose with his hands until she stopped crying, police said. He then put her in car seat that was inside the apartment.

Their mother, Gloria Yanes, came home at 5:30 a.m. Friday and thought the little girl was sleeping, according to police. Just past 6 a.m., she tried to wake Larissa to feed her and found her unresponsive, according to police.

Authorities wouldn’t say Saturday if Aguiluc had said why he killed his sister or why he placed her in the car seat.

He is scheduled to have bond review in Montgomery County District Court Monday afternoon.