(CBS NEWS) -- Despite years of public health campaigns, many American parents are still putting their babies to sleep in an unsafe position, a new study finds.

The study found that just half of mothers surveyed said they always put their babies to sleep on their backs.

Experts called the findings "frustrating," since back-sleeping has long been promoted as a key way to cut the risk of sudden infant death syndrome (SIDS).

In 1994, the U.S. government launched the "Back to Sleep" campaign to encourage parents to put their babies to sleep lying face-up. That came after research identified tummy-sleeping as a major risk factor for SIDS.

Researchers believe SIDS is related to problems in the brain regions that control breathing and arousal from sleep.

MORE: Read more at CBSNEWS.com