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

At one point, after the infant stopped crying, the teenager carried her in his arms around their apartment, placed her in a car seat that was in the living room, retreated to his bedroom and waited for their mother to return from an overnight job, according to authorities. The suspect, identified by police as Johnathan Aguiluc, has been charged as an adult. He was being held without bond on Saturday in the Montgomery County jail.

Early Friday morning, 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. Paramedics treated the child and took her to Holy Cross Hospital, where she was pronounced dead at 6.54am. Police identified the girl as Larissa Yanes and said she lived at the apartment. Detectives observed injuries on the child that they determined could not have been self-inflicted, police officials said. They learned that the night before, the 39-year-old mother of the teenage boy and infant girl left for work at 10.30pm. According to police, the girl was being watched by her brother.

Detectives interviewed Aguiluc, and he said he had beaten his sister, which caused her to cry, according to Capt Paul Starks, a police spokesman. Aguiluc also said he covered his sister’s mouth and nose with his hands until she stopped crying, police said. He then put her in the car seat in the apartment. Their mother, Gloria Yanes, arrived home at 5.30am on Friday and thought that the girl was sleeping, according to police. Just past 6am, she tried to wake Larissa to feed her and found her unresponsive.

Authorities on Saturday would not say whether Aguiluc had said why he killed his sister or why he placed her in the car seat. Online court records do not indicate whether a defence attorney has been retained for Aguiluc. He is scheduled for a bond review in Montgomery County District Court on Monday afternoon, when prosecutors are expected to provide further details about the case.