ANGELES NATIONAL FOREST — Firefighters have corralled 83 percent of the 4,200-acre Williams Fire, which ignited in the forest north of Glendora one week ago.

The final uncontained borders of the wildfire were in the steep slopes of the Sheep Mountain Wilderness Area, U.S. Forest Service officials said Sunday.

“Last night only smoldering was observed with no visible flame,” USFS officials said in a written statement.

“Along the northern portion of the fire, crews will continue to hold and improve fire line as well as mopping up,” the statement said. “The southern portion of the fire will be on mop-up and patrol status.”

Five-hundred and sixteen firefighters and nine water-dropping helicopters remained assigned to fight the fire.

When the fire broke out on the afternoon of Sept. 2, in addition to dozens of residents of Camp Williams and a nearby drug and alcohol rehabilitation center, officials evacuated more than 12,000 people who had gone to the San Gabriel Canyon to relax during the Labor Day weekend.

The permanent residents were allowed to return to their homes Thursday.

Sheriff’s deputies Saturday escorted the evacuated day-campers who had left belongings behind during the hurried evacuation back into the forest for one hour to collect their things.

Full containment of the fire was expected Thursday.

The cause of the fire remained under investigation, however sheriff’s deputies had initially received a report of a car on fire near Camp Williams before the wildfire erupted.

During the first day of the blaze, a completely charred compact car was removed from the forest on the back of a flatbed tow truck.