The Federal Aviation Administration and the National Transportation Safety Board are investigating an explosion in a hot-air balloon Saturday in Temecula.
The blast injured four passengers, three of them seriously, officials said.
One of the passengers was sent to a burn center and two were taken to trauma centers, the Riverside County Fire Department said. A fourth person suffered minor injuries, and a fifth passenger was not injured.
Riverside County Fire Department spokesman Mark Annas said the balloon landed safely, "and then, there was an explosion." Authorities were called about 7:45 a.m.
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