An Associated Press analysis has found that nearly 1,400 water systems serving 3.6 million Americans exceeded the federal lead standard at least once during the past three years.

The affected systems are large and small, public and private. They include 278 systems that are owned and operated by schools and day care centers in 41 states.

The AP reviewed a quarter century of sampling data reported by 75,000 drinking water systems that are subject to the U.S. Environmental Protection Agency's lead rule.

Water agencies must inform customers about the problem and take steps if more than 10 percent of sampled homes are above the EPA's limit of 15 parts per billion. The most common way to address high lead levels is to add chemicals to control corrosion and prevent leaching.

Responding to the crisis in Flint, Michigan, school officials across the country are testing classroom sinks and cafeteria faucets for lead, trying to uncover any concealed problems and reassure anxious parents.

Just a fraction of schools and day care centers are required to check for lead because most receive their water from municipal systems that test at other locations. State and federal lawmakers have called for wider testing.

Among schools and day cares operating their own water systems, U.S. Environmental Protection Agency data analyzed by The Associated Press showed that 278 violated federal lead levels at some point during the past three years.

Roughly a third of those had lead levels that were at least double the federal limit. Most problems can be traced to older buildings with lead pipes.