Maldonado said Friday that the explosion was caused by a gas pipe rupture but added, "We don't know what caused [the rupture] or what happened."
"We will find out soon," he said.
The explosion occurred just after 6:15 p.m. Thursday in a residential area near highways 280 and 380 in San Bruno, just south of San Francisco. Thirty-eight structures were completely destroyed and another seven badly damaged.
"You've heard the numbers," San Bruno mayor Jim Ruane said Friday in a news conference. "Unfortunately, the numbers are going to get higher."
Local news reports said residents had attempted to alert Pacific Gas and Electric, the company that operates the pipelines, to the smell of gas days before the explosion.
"We have records that we are going back through right this minute to try and confirm what those phone calls looked like and when they occurred," PG&E president Chris Johns said Friday. Johns said that company policy was to immediately respond if someone calls in with a complaint about the smell of gas.
"We're really saddened and sorry about this tragedy," he said.
Officials from the National Transportation Safety Board are investigating the cause of the blast and will work with PG&E to determine exactly what happened.
"I want to make sure everybody knows that we are committed to do what's right and what's appropriate to help all the families and others who have been impacted by this tragedy," Johns said.
Johns said that no PG&E crews were working in the vicinity during the explosion, but he did not know about any other construction going on. The pipe that ruptured, Johns estimated, was 40 or 50 years old.
ABC News' Ariane Nalty, Neal Karlinsky and The Associated Press contributed to this report.