The country’s fourth-largest city ordered millions of residents Sunday night to boil their tap water, citing a morning power outage at a city water purification plant.

The city’s school system also announced late Sunday that it would be closed Monday, at a minimum, as officials “closely monitor the situation.” The district’s 276 schools serve nearly 200,000 students. Other smaller school districts in the area also announced closures.

The 10:30 a.m. outage caused water pressure at the plant to drop below required levels. That caused water pressure problems for some residents that were resolved later in the day, public works officials said in a statement.

More broadly, however, the outage called into question the safety of water for 2.2 million customers. Houston’s order Sunday night directed residents to boil all water used for food, drinking, bathing or tooth brushing and to avoid using water from refrigerators or ice makers.