Authorities are investigating unauthorized transfers from bank accounts kept by Crystal Lake Elementary School District 47, police confirmed Friday.
District 47 announced the investigation more than one week after a district staff member discovered the transfers.
Police would not say how much money might have been taken.
"There has been a substantial amount of financial loss," Crystal Lake Deputy Police Chief Dennis Harris said. "We're not going to release the details of that as that may affect our investigation."
Harris would not reveal which accounts had been compromised, how many unauthorized transfers took place or the time period during which the suspected crimes might have been committed.
"We're still in the early part of the investigation," Harris said. "There is a review of all the transactions - to see the scope of what has taken place."
Police have no suspects and no one in custody, Harris said.
Because bank deposits are guaranteed by the Federal Deposit Insurance Corporation, the FBI is also working on the case.
In a news release, district officials said they have put additional safeguards in place to stop further losses and plan to review the district's accounting practices.
"We've plugged the leaking boat," Harris said. "We're still trying to figure out how the hole got in the boat in the first place."
District 47 officials could not be reached for comment Friday afternoon.