The Cobb County school system was forced to spend nearly $20,000 to replace 18 catalytic converters stolen earlier this month, the district said Monday.

A total of 22 catalytic converters were stolen from maintenance vehicles July 5, but only 18 of those were replaced, Doug Goodwin, district spokesman, told Channel 2 Action News. Four vehicles were not returned to service.

"As soon as the maintenance came in and started their trucks up, they discovered it," Charles Rogers, one of the district's investigators, told Channel 2.

Rogers said school police officers got some finger prints and are following up with local companies such as recyclers, where thieves may have taken the parts for quick cash. The precious metals inside the catalytic converters are the target, according to police.

This is the second time in less than a year that the school district has been hit by the metal thieves, possibly pointing to a need for tighter security in the truck lot off Glover Street in Marietta. On July 25, 2011, five catalytic converters were stolen from school system trucks, costing the district $4,186.56 to repair, Goodwin said.

"We hope we can do something to prevent it the next time, so there won't be a next time," Rogers said.

Jeff Timbs at Canton Road Muffler and Automotive said catalytic converter thefts were a major problem a few years ago, but media coverage of the crimes deterred some of the would-be thieves. Recently, that has changed, Timbs said.

"As a matter of fact, it almost stopped for a while, at least around this area, but it's picking back up," Timbs told Channel 2.

Anyone with information about the recent thefts is asked to contact the school system at 770-426-3300.