U.S. National Forest officials said they were investigating the cause of a brush fire that charred 75 acres and shut down Interstate 5 near Castaic Wednesday.

The blaze began about 4:40 p.m. Wednesday near Templin Highway in the Angeles National Forest north of Castaic.

“This is the fourth year of very low rainfall (and) this is the largest fire in our forest this year,” said Stanton Florea, a spokesman for the National Forest. It took about 140 firefighters more than four hours to douse the blaze. Six helicopters and four air tankers were called in.