The 15-year-old and 17-year-old brothers, whose names have not been released, are accused of murdering Autumn Pasquale. After a two-day search, Pasquale's body was found in a recycling bin near where the boys lived.
Authorities did not say what was in the Facebook post and it appeared not to be publicly available by Tuesday evening. Investigators said the brothers also left behind evidence inside their home, including the girl's bike and backpack.
The brothers turned themselves in Tuesday and were charged with first-degree murder, conspiracy to commit murder, disposing of a body, tampering with evidence and theft. The younger boy is also charged with luring.
The boys have been charged as juveniles, but prosecutors said it's likely they'll ask that the case be moved to adult court.
The brothers are expected to appear in court Friday.
The Associated Press contributed to this report.