OTAY MESA (CBS 8/CNS) - A fire at a metal scrap yard next to Brown Field on Otay Mesa sent a thick black smoke plume into the sky that could be seen for miles Sunday and took firefighters more than four hours to douse.

Firefighters were dispatched to the fire at about 12:45 p.m. in the 900 block of Heritage Road near Pogo Row and immediately called for a first alarm response, according to San Diego Fire-Rescue Department spokeswoman Monica Munoz.

Firefighters had to be careful in fighting the fire, because they weren't sure what exactly was burning.

The fire was contained to an area of about 400 square feet, Munoz said.

No injuries were reported and the cause had not been determined as of late afternoon Sunday.

General aviation flights at Brown Field were not affected by the fire.