Cops in St. Louis are mystified by the theft of yellow school buses in recent months.

"It is unusual when you have a situation to where a school bus is stolen and remains missing," said Srgt. James McWilliams of the St. Louis County Police Department.

At least one bus has been stolen every month since September, in addition to a bus that was stolen in May, bringing the total number of bus heists to eight. The buses were stolen from two private schools, a public school, and from two locations owned by a company that contracts transportation.

Police from St. Louis County and adjacent Jefferson County notified the FBI as is common practice whenever large vehicles are stolen. According to McWilliams, the FBI said that the thefts were not a threat and instead seemed to be isolated incidents.

A brand new bus can cost up to$60,000 and  investigators don't have any leads as to why the buses were stolen or where they are being hidden. Jefferson County police caught one bus being driven out of a Windsor School District lot. The perpetrators had clipped the padlock, fixing a new lock on the gate, though they did not leave a key. Some officials speculate that the metal parts may be used as scrap metal since many of the parts are interchangeable.

For Brian Ryherd, the principal of Lutheran South High School, the theft of his bus is more a cause for consternation than desperation.

"It's more annoying than anything," Ryherd said.

The school's bus was not brand new and will cost around $10,000 to replace. One of five buses at the school, it was used to transport the students to extra- curricular activities and to field trips. Ryherd, along with police, said that the thefts were difficult to pinpoint because of when they occur. Often the buses would be there when the doors closed on Friday and gone when they opened on Monday.

Police in both counties have increased security and are asking police and citizens to remain alert.  Anyone with information is asked to contact St. Louis County Detective Kevin Funston at 314-615-8622 or the Jefferson County Sheriff's Office at 636-797-5515.