Houston’s public school system will be closed again Tuesday following a power outage at a water treatment plant on Sunday that has forced the entire city to boil water before it can be used. Water officials had expressed hope that the boil water notice for the city’s 2.3 million residents would be lifted by early Tuesday but that hasn’t happened yet.

The power outage at the East Water Purification Plant early Sunday occurred when a transformer failed, according to local TV channel ABC 13. The backup transformer and backup generator both failed to kick on, for some unknown reason.

“We do know that the two transformers malfunctioned. One was a backup to the other. It was just one of those unique circumstances in which you have these transformers that malfunction and then you have the backup that malfunctioned,” Houston Mayor Sylvester Turner told ABC 13.

The city of Houston has a $56 million contract with NRG Energy Services for backup power at the East Water Purification Plant, but it’s not clear why the backup power failed to kick on. NRG did not immediately respond to a request for comment early Tuesday.

Power outages at water treatment facilities can cause a loss of water pressure, something that happened roughly a half hour after the power failed on Sunday morning, according to the Wall Street Journal. Fourteen of the power plant’s 16 sensors dipped below “emergency” levels for about two minutes, while the remaining two sensors went below “emergency” levels for 30 minutes, according to the Journal.

Loss of water pressure at a water treatment facility can allow pipes to reverse, potentially letting in contaminated water, though city water officials still believe that didn’t happen in this case. But the proper tests to be sure can take 18 hours, forcing the entire city to come under a boil water notice just to be safe.

“Due to the Boil Water Notice issued by the City of Houston Sunday evening, all Houston ISD campuses and facilities will be closed Tuesday, Nov. 29.,” the school district said in a tweet.

“This decision has been made due to the logistical challenges caused by the notice. Those challenges prevent the district from being able to provide meals for its students and ensure safe water is available for students and staff,” the school district continued.

“All HISD employees will be working remotely unless otherwise instructed by the chief of their business area. While students are not on campus, we encourage them to engage with digital academic resources that are available 24/7.”

Texas Governor Greg Abbott issued a statement on Sunday saying that he was directing state resources to Houston but claimed that three power plants had actually lost power. It’s not immediately clear why there was a discrepancy between the governor’s announcement and the city’s.

Click through for more photos from Houston during the boil water notice.