The apartment's two residents were not home at the time of the blaze, he said.
"It was a quick and aggressive attack," he said.
The fire was extinguished before it spread to neighboring units, but smoke and water damage to the apartment below caused the temporary evacuation of a couple living there, he said.
The blaze above gutted the top-floor apartment's kitchen and living room, rendering the unit uninhabitable, he said.
No one was injured during the fire and arson investigators were at the scene Tuesday evening to determine its cause, he said.
The Glendale-Crescenta Valley Chapter of the American Red Cross provided immediate assistance to the four residents, who were traumatized and shocked when they arrived on scene, said Robert Reynoso, director of emergency services for the Glendale Red Cross.
The residents will receive free temporary housing and other immediate necessities, including food, medication and clothing, he said.
The couple living in the water-damaged bottom unit may be able to return within a few days, Reynoso said.
All four residents will receive counseling on how to recover, financially and emotionally, from the fire, he said.
"It gives them a chance to have a base from where to initiate the recovery process," he said.
Firefighters estimated damage to the apartment building at about $125,000, with an additional $25,000 in lost property, according to a Glendale fire report.
The apartment blaze came after the skies were already darkened with smoke and ash from a brush fire in Griffith Park, just west of Glendale.
As of 9:25 p.m., the fire had burned up to 200 acres in the mostly secluded, steep slopes and ridges of the 4,000-acre park and was up to 25% contained, said Melissa Kelley, Los Angeles Fire Department spokeswoman.
Three Glendale engines and one battalion chief were called in to assist Los Angeles firefighters control the blaze and protect structures, Guerrero said.
Five water-dumping helicopters were used throughout the day to help fight the blaze, the cause of which was still under investigation, Kelley said.
JASON WELLS covers public safety and the courts. He may be reached at (818) 637-3232 or by e-mail at jason.wellslatimes.com.