WASHINGTON -- Federal regulators are changing course on a law aimed at keeping swimmers, especially children, from getting trapped in pool drains.

The new guidance requires public pools with a single main drain to have a back-up system that could shut the suction of the drain. Previously, operators of those pools were told they could fit their drains with a special cover designed to prevent entrapments.

The move means thousands of public pools in cities and towns will have to take a fresh look at their drains, and probably have to buy costly back-up systems. Some may close if they don't have the new equipment by next May.