SAN ANTONIO -- A 12-year-old girl was missing earlier than police were told, law enforcement authorities said Thursday afternoon.

Rosa Sandoval was actually missing around 11:30 p.m. on May 27 instead of the morning of May 28 when she was walking to school, San Antonio Assistant Police Chief Tyrone Powers said at a news conference.

Powers said Sandoval's sister told them the two got into an argument over a glass of tea before the girl left on her own.

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"Rosa went into the bedroom. Her sister went to bed 30 minutes later and Rosa was gone. And she has not returned," Powers said.

The missing girl's sister came forward with the new information because her conscience was bothering her, Powers said.

The new information will change the way the case will be investigated, Powers said.

"We eliminated a lot of people as suspects," Powers said. "We have to now go and re-eliminate them as suspects because an alibi for 7:30 a.m. is not the same alibi for 11:30 p.m."

The development comes two days after the FBI offered a $50,000 reward for information that leads to the discovery of Sandoval.

The reward is in addition to a $5,000 reward Crime Stoppers is offering in the case.