SAN MARCOS -- One person died and four other people were injured in a fatal fire early today at a San Marcos apartment, authorities said.

Someone reported the blaze in the 500 block of East Barham Drive at 3:29 a.m., said sheriff's Lt. Tony Giammarino.

He said the person kept updating information, later reporting the second-floor apartment was completely on fire with at least one person trapped inside.

The sheriff's arson and bomb unit was called to investigate, and a body was found inside the apartment, Giammarino said.

"There were six occupants in the apartment," said Battalion Chief Rick Vogt of the San Marcos Fire Department. "No other apartments were damaged. The surrounding apartments were evacuated."

As the investigation continued this morning, officials said two people from the apartment were taken to the UCSD Medical Center in San Diego, one by air ambulance.

Two others were taken to Palomar Medical Center in Escondido for treatment of smoke inhalation, they said.

The cause of the fire was still under investigation.