Houston - More than 2 million people in the Houston area were under a boil order notice Monday after a power outage at a purification plant caused water pressure to drop, and the mayor of the nation's fourth-largest city ordered a full review of the system.

The notice tells customers to boil water before it's used for cooking, bathing or drinking. Multiple Houston-area public and private schools, as well as some local colleges, were closed Monday as a result of the notice, while others made adjustments to provide affected campuses with bottled water and sanitizer.

The notice was issued Sunday, hours after two transformers failed, causing power outages at the water plant, Houston Mayor Sylvester Turner said at a press conference Monday. There was no indication the water system had been contaminated.

Water quality testing was underway, Turner said. He said he expects the notice to be lifted by early Tuesday at the latest, once the state's environmental agency gives an all-clear after analyzing test results.

According to Turner, the city issued a notice, which affects all of Houston and multiple adjacent areas, in an 'abundance of caution' after the two transformers - a main one and its backup - 'uniquely and coincidentally' failed. The problem affected the plant's ability to treat water and pump water into the transmission system, resulting in low water pressure.

The power system at the water plant undergoes regular maintenance, Turner said, but he did not give a timeline for how often. The mayor said he has ordered a diagnostic review of the system to understand how this was possible and how it can be prevented. He said because the issue was within the plant's system, backup power generators would not have made a difference.

Sixteen sensors marked dips under the minimum pressure levels required by the Texas Commission on Environmental Quality - 14 of them for only 2 minutes and two of them for nearly 30 minutes, Turner said.

Untreated groundwater can enter a water system through cracked pipes when water pressure drops. Customers are told to boil water to kill bacteria that could be harmful.

'We are optimistic the results will come back clean,' Turner said.

Turner defended the decision to warn residents about the water quality several hours after the issue first occurred and apologized for the disruptions to businesses, schools and elective surgeries. He said the dip in pressure did not automatically trigger a water boil notice, but a decision was made to issue one based on the data once the city consulted with and was instructed to do so by TCEQ.

Water infrastructure and quality has been a prominent issue in cities large and small throughout the U.S., including Baltimore; Honolulu; Jackson, Mississippi; and Flint, Michigan.