When one school bus disappeared, police wondered who would steal it. Then another went missing, and another, until the total reached eight and the mystery deepened.

Were they sold for scrap metal? Stripped for parts? Converted into hunting cabins? Taken out of the country?

Whatever the explanation, at least one full-size regulation yellow bus has disappeared each month since September — plus one in May — from five locations in south St. Louis County or Jefferson County.

None of the buses has been found.

"Why would you take a school bus, and where do you hide a school bus?" asked Jefferson County sheriff's Capt. Ron Arnhart, speaking for vexed investigators.

"It's not your usual stolen vehicle," said Sgt. Tom Naughton of the St. Louis County police Auto Crime Unit. "Most vehicles come up recovered, and not having these school buses surface anywhere is unusual too."

The FBI was notified, Naughton said, which is standard when a particularly large vehicle — or one that a terrorist might use for access to a sensitive area, such as a firetruck or school bus — is taken.

"We have not received any information back from them about national concerns, and this appears to be isolated," he said.

Officials said that new, each bus is worth about $60,000.

Exactly when these were stolen is hard to pinpoint because they went missing on weekends or when school was otherwise not in session. The victims were two parochial schools, a public school and two locations belonging to a company that contracts for transportation.

Lutheran South High School expects to spend about $10,000 — that wasn't in its budget — to replace the one it lost, which was about 10 years old.

"We came in on a Monday morning and there were four buses instead of five," said Principal Brian Ryherd. "It was just gone. ... It's not a horrifying expense, but it's just another problem. I don't know what someone would do with an old school bus."

Lutheran South uses the buses to take students to extracurricular activities, such as basketball, wrestling and swimming tournaments. The missing one was the only handicapped-accessible bus. "We don't have any kids that need it at the moment, but it is certainly nice to have," Ryherd said.

The four buses stolen from First Student's lot in St. Louis County were retired and not in use, so there was no impact to students, said Jen Biddinger, a spokeswoman for the private contractor.

"The thefts are highly unusual, and not something we are seeing companywide, but I am told it is a real problem in the Affton area," Biddinger said. "We have increased security at the site."

Naughton said St. Louis County police had stepped up patrols as well.

"When the break in this case comes, we'll be able to clean them all up because it's such an odd thing to steal," he said. "It's a challenge to investigate, but we're working every angle we can work."

The most recent theft is so far the only one at least partially caught on video. It just shows a bus driving away from the Windsor school bus lot in Imperial about 3 a.m. Saturday.

In that case, Jefferson County deputies believe the thieves used bolt cutters to sever chains securing the gates and then fastened them again with new padlocks.

"By making it look like everything is OK and not having a missing chain, it might have taken an officer longer to detect," Arnhart explained. The locks and chains are being processed for DNA.

Naughton said several parts believed to have come from a school bus turned up in a scrap yard in Washington County but could not be linked to the missing vehicles.

"The problem is, a lot of these parts are interchangeable," he said. "They are International and Blue Bird models. International front ends are interchangeable with International trucks."

Anyone with information is asked to contact St. Louis County police Detective Kevin Funston at 314-615-8622 or the Jefferson County Sheriff's Office at 636-797-5515.