Houston officials are investigating the cause of the electrical failure that left millions of people without clean water for almost two days and closed local schools. Investigators were preparing to descend into manholes at the city’s biggest water-treatment complex to examine cables suspected of being a possible source of Sunday’s calamitous outage, Erin Jones, a spokesperson for the Houston Public Works Department, said on Tuesday. Those inspections could be delayed because of expected thunderstorms that would make it hazardous to go underground, she said.