A burglary ring that loots school classrooms of expensive computer gear appeared to be at work as thieves hit the Marin Country Day School in Corte Madera on Thursday night, taking 18 laptop computers in the second heist at the school in a week.

The theft came after a similar burglary at the school in which 20 laptops were stolen — and follows a rash of computer thefts at other Marin schools.

School officials remained mum Friday.

"We had another incident last night," said Ann Borden, director of communication at the school at 5221 Paradise Drive. She declined further comment, saying police advised the school "not to give out any information" pending an investigation.

Twin Cities police Detective Michael Mejia said officers across the county "are looking at a lot of possibilities" after a rash of school computer thefts. Eighteen laptops were taken in the latest incident, he said, adding investigators were still at work on their report.

Last week, officials discovered that an electronic gate at Marin Country Day School was broken and the rear door to a classroom pried open by night-time thieves who took 20 silver 13-inch Apple MacBook Pros valued at a total of $24,000.

In addition, authorities are investigating the theft of 20 laptop computers valued at $26,000 from the Strawberry Point School library. A locked metal security cart containing the Apple MacBook laptops was reported missing about two weeks ago by Principal Leslie Thornton.

In that case, it appeared that a library door was pried open, said Marin County sheriff's Lt. Barry Heying. A lock was sawed off a gate in front of a garbage service driveway on the campus, enabling vehicle access.

A hiker later found the empty metal cart on a trail near Panoramic Highway. The cart was heavy, requiring two people to lift.

Old Mill School in Mill Valley had dozens of laptops stolen from its multipurpose room in July during a summer construction project.

Investigators in several jurisdictions are sifting through "our usual suspects" in an effort to find the culprits, Detective Mejia said. "It may be someone from out of the county," he noted.