She is described as Mexican American, about 5 feet, 3 inches tall, with reddish brown hair and braces. She also has a piercing on her lower lip.
Monica's mother said she dropped her daughter off at school last Thursday morning, but when she didn't come home that afternoon, her parents became concerned.
Monica's sister was able to reach her by cell phone, and she said she'd be home in five minutes, but she never arrived.
The family passed out fliers with Monica's photo, and then received a call from a man who said he saw the teen at an apartment in Los Angeles. Police followed the lead but found nothing.
Authorities said Monica ran away from home once in February, but this time, they do not think she ran away.
"We have no evidence that indicates she is a runaway at this time," said Capt. Anthony Miranda of the Bell Police Department.
Anyone with information is urged to call the Bell Police Department at (323) 585-1245.