For the second night in a row, Oceanside firefighters battled brush fires burning along the San Luis Rey Riverbed, NBC 7/39 reported.
On Tuesday, the Camp Pendleton Fire Department assisted Oceanside firefighters in battling three separate fires that merged into one big fire.
They were called to the scene at about 6:30 p.m.
Nearly 100 firefighters battled the blaze in rough terrain as the fire burned between five to 10 acres.
No structures were threatened and no one was injured, officials said.
A large perimeter was established around the fires in an effort to catch those who might be setting the blazes, Oceanside police said.
Police said they have stepped up patrols in the area following a rash of brush fires.
More than a dozen fires have been reported in the last two months, with the most recent Monday night, at State Route 76 and North Canyon Drive, investigators said. Firefighters were able to extinguish Monday's fire within minutes.
Firefighters and Oceanside police are now investigating these fires that have all burned within 1.25 miles of each other. So far, they said they have no evidence to link the fires together, other than location.
Fire officials said the area is known as a popular spot for transient camps, and that it has a history of small fires.