Investigators today are looking into the cause of a two-alarm fire that destroyed a mansion under construction in Eastlake, the chief of the Chula Vista Fire Department said.

The 6500-square-foot residence was fully involved with flame when firefighters arrived around 10:20 p.m. Saturday at 2890 Gate 15 of Woods Drive, so they immediately called in help to protect surrounding structures, Chief Dave Hanneman said.

With the home under construction, the flames were able to move quickly through the exposed wood frame, Hanneman said.

The chief said it took about a half-hour to extinguish the flames, and firefighters remained on scene throughout the night to guard against hot spots or collapse of the structure.

No one was hurt, the cause of the fire was not immediately known, and a damage estimate was unavailable early today, he said.