LANHAM, Md. (CBSDC)- A Montgomery County teen has been charged with first-degree murder after admitting to police that he beat and smothered his 7-month-old sister while his mother worked a night shift Thursday evening into Friday morning.

Johnathan Aguiluc, 14, is being held without bond and is due in a Rockville courtroom Monday. Authorities say he was charged as an adult.

A police investigation revealed that 39-year-old Gloria Yanes, of the 11600 block of Lockwood Drive in White Oak, went to work at 10:30 p.m. Thursday evening. She returned home at 5:30 a.m. Friday, and saw that her daughter, Larissa Yanes, appeared to be sleeping in a car seat. Shortly after 6:00 a.m., Yanes attempted to wake the baby to feed her and found her unresponsive.

The child was taken to Holy Cross Hospital, where she succumbed to her injuries.

Detectives who interviewed Aguiluc say he admitted to beating his sister, which made her cry, then covered her mouth and nose area with his hands until she stopped crying.

A preliminary hearing in the case is scheduled for Feb. 22.

The investigation is ongoing.

Follow WNEW on Twitter.