FBI investigates deadly building explosion

The blast in a Southern California medical building killed one woman and injured at least three other people.
2:20 | 05/16/18

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Transcript for FBI investigates deadly building explosion
Robin. To that explosion that left one person dead in California. You can see the extensive damage inside. ABC's kayna Whitworth is there on the scene for us this morning. Kayna, the FBI and homeland security, we know they are investigating. Reporter: Yeah, robin and the Orange county sheriff's office is taking the lead on this investigation as they try to determine the cause of this explosion, this morning questions are swirling as authorities failed to rule anything out. Could it have been a ruptured gas line or perhaps something much more sinister? A deadly explosion rocking a southern California community. I need to have the building evacuated. All crews pull back from the incident. Reporter: Chaos just after 1:00 P.M. In the first floor of this medical building. Plumes of white smoke pouring out. Debris sent flying into the parking lot. First responders racing to help. The FBI, ATF and bomb squad combing the scene for answers. Nearby witnesses describing the horrendous blast. We heard a very loud boom and I actually felt the building shake. Saw the fire come out on one side, the insulation on the other popping out and debris all over the floor. Reporter: These photos inside the building showing the breadth of the many damage. Sheriff's deputies in gas masks looking for clues killing one woman and at least three other injured. An official saying the explosion was believed to be an intentional act. We have not found any type of specific device inside of the building right now that could tell us or lead us to what exactly the device was if it was a device. Reporter: People in nearby buildings forced to evacuate including dozens of small children across the street. Leaving the building holding hands, some wheeled out in their cribs. To have to do something like that with those little kids had to be scary. It's a tragedy. Any time you're near this or you see it, it's going to affect you. Reporter: So I speak with a former FBI special agent who said if it was intentional then we have a bombmaker who attacked their victim and that is a very dangerous situation. All authorities will confirm right now is that they're analyzing surveillance video. Robin. Dangerous and frightening. All right, kayna, thank you.

This transcript has been automatically generated and may not be 100% accurate.

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