Carrie Braun, an Orange County Sheriff's Department spokeswoman, said authorities were working to determine if the explosion was an accident or an intentional act, the newspaper reported.
One woman is dead and three other people are injured after a mysterious explosion rocked a medical building in Southern California. One of them reportedly said "take care of my mother, take care of my mother". Tony Bommarito and sheriff's officials said later that was not the case.
Associated Press writer Michael Balsamo in Los Angeles contributed to this report.
Tony Dik, whose child attended the day care, told NBC that parents were anxious and emotional when they found out that the preschool was on the same road as the explosion.
There's no "initial indication" of terrorism right now, Mike Gifford, an Federal Bureau of Investigation spokesman in Los Angeles, said, though authorities will continue to investigate possible terrorism out of caution.
"It's disgusting, it's a tragedy", Orange County Fire Authority spokesman Tony Bommarito told ABC News. Dave Sawyer said Tuesday night that the size of the explosion makes it suspicious.
Shin said he and others hurried down the stairs to get out of that building, and saw "two holes in the wall" of the medical facility. McWilliams says one woman had skin peeling on her arm. The opposite girl was bleeding from her head. The extent of the injuries was not immediately known. "Investigation is in early stages, as information is confirmed we will share it", the department said in a tweet.
The identity of the female victim killed has not been released. The person who died was inside the building at the time of the blast, officials said. The reason for the blast is being investigated.
There was no immediate information about the person who was killed. All were taken to a hospital, she added. All surrounding buildings have also been evacuated. They were being reunited with parents at a nearby store parking lot.
The explosion was reported before 1:25 p.m.at a two-story office building located at 11 Mareblu.
Chief Brian Fennessy detailed three other adults have been hospitalised following the horrific blast.