Babies are twice as likely to die of sudden infant death syndrome in the winter than in the summer and at age 2 months than age 4 months, a new federal study shows.

But even though SIDS experts long presumed those facts to be true, they still don't know why.The national Centers for Disease Control, analyzing eight years of infant mortality data, reported Thursday on the SIDS risk.

The findings are important to researchers looking for the cause of SIDS, which kills about 6,000 American infants a year.

"What we suspect is that an infectious process is somehow triggering death in these infants during cold weather," said Dr. Jose Becerra, an infant health specialist.

"There's also a remarkably similar pattern (for age and season) in infectious diseases - very similar."