Published at 4:50 a.m.: Police have charged a 14-year-old boy as an adult with first-degree murder in the death of his 7-month-old sister.

Johnathan Aguiluc, 14, of White Oak, Md., is being held without bond in the death of Larissa Yanes.

Investigators in Montgomery County say that the girl's mother, 39-year-old Gloria Yanes, left her apartment in the 11600 block of Lockwood Drive at 10:30 p.m. Thursday to go to work, leaving Aguiluc to watch Larissa.

During the evening, detectives say that Aguiluc beat Larissa, which caused her to cry. The teen then said he covered Larissa's nose and mouth with his hands until she stopped crying and placed the baby in her car seat.

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Gloria Yanes returned to the apartment at around 5:30 a.m. Friday. Around half an hour later, she attempted to wake Larissa up to feed her.

When Larissa did not respond, her mother called the police and fire/rescue. Larissa Yanes was taken to Holy Cross Hospital, where she was pronounced dead of her injuries shortly before 7 a.m.

Aguiluc is scheduled to appear in court for a bond review on Monday.

By NBCWashington.com