Police in California have admitted they are baffled by a series of car thefts where robbers use a small hand-held electronic device to unlock supposedly secure car-locking systems.

"This is bad in the sense we're stumped," Long Beach deputy police chief David Hendricks told NBC. "We are stumped and we don't know what this technology is."

The police force has taken the unusual step of releasing video of two recent car break-ins in the southern California town by robbers using the device, which resembles an electronic key fob. In both cases the device unlocks the passenger side door and appears to disable the alarm system, allowing the thieves to rummage through the car but not drive it off.

Hendricks said that the force has been in discussions with car manufacturers and mechanics to try and find out what the device is, but so far have had no luck and are appealing to the public for information.

"This is really frustrating because clearly they've figured out something that looks really simple and whatever it is they're doing, it takes just seconds to do," said security consultant Jim Stickley. "And you look and you go, 'That should not be possible'."

Cracking electronic car keys is perfectly possible. Back in 2011 a Swiss team of researchers opened and started cars from eight different manufacturers using inexpensive hardware.

The team positioned one aerial next to the target car and another within eight meters of the car key itself. By transmitting data between the two using both wired and wireless communication the car could be unlocked and disarmed.

More recently, El Reg reported last year on a $30 kit that can be used to hack cars using the On-board diagnostics (OBD) computer system. The system was being used in Britain to scan and copy key fobs for high-end BMW vehicles.

This latest device looks like it could be somewhat similar, but it's not known if the car thieves staked out the two robbed Long Beach cars in the police video to copy their owner's keys, or if this new device is the car robber's dream: a universal access key that works on most models.

California police have advised motorists to empty their car of all valuables at night – including garage door openers that could be used to burgle a property. ®