MODESTO (CBS13) – Firefighters responded Thursday afternoon to a large fire at a wrecking yard in Modesto.

The fire started around 1:30 p.m., sending up thick, black smoke. When crews arrived at the business on Crows Landing Road, they found 20 vehicles on fire.

Traffic on Highway 99 was reduced to a crawl.

Authorities were worried flames would spread to nearby businesses or homes; however, that didn’t happen.

Four firefighters had to be treated for heat-related injuries.

People familiar with the area say homeless people have set fires in the area in the past. Firefighters haven’t said whether that was the cause of today’s blaze.