NEW YORK – All those sophisticated theft-prevention devices won't save your car if you leave the keys inside, a lesson that more Americans are learning the hard way.

U.S. car thefts with keys left inside jumped 14 percent from 2012 to 2014, reaching 44,828 last year, according to a study issued Monday by the National Insurance Crime Bureau. The actual number is probably higher because some people don't admit their carelessness to police or their insurer, the NICB said.

"Stealing a vehicle is very difficult with today's anti- theft technology, and leaving the keys in the vehicle is an open invitation for the opportunistic car thief," NICB Chief Executive Officer Joe Wehrle said in a statement.

Criminals are otherwise having a harder time stealing cars because of devices like smart keys and fuel pump disablers designed to prevent hot-wiring. About 660,000 cars were stolen in the U.S. last year, according to NICB, which uses data from the FBI. That's an 8.5 percent decline since 2012.

The NICB advised drivers to stay with their cars when giving the vehicles time to warm up on winter days, and to bring the keys along even on short trips to convenience stores. Some people who leave keys behind may be hoping to have their car stolen to collect the insurance, the group said.