"We think it's possible it's an arson, but we don't have a cause yet," Jason Laughlin, spokesman for the Camden County Prosecutor's Office, said of Sunday's blaze at the former Concord Chemical Inc.
Investigators are reviewing surveillance video from a nearby business, according to sources close to the probe.
A gas station attendant on Admiral Wilson Boulevard said a Camden police detective had stopped by Sunday morning asking whether anyone in a white car or van had filled up gasoline containers.
The man, who asked not to be identified for fear of reprisal, said two men had filled a small container around 2 or 3 a.m. Sunday.
"There's been a lot of fires in a short period of time," the attendant said. "Something funny is going on."
The theory that all three are the handiwork of a serial arsonist follows a news conference last week in which Camden Mayor Dana L. Redd voiced concern over the short time frame in which the first two fires occurred.
Causes of those fires - June 9 at a former tire warehouse in the city's Gateway section and on June 11 at an abandoned garment factory in the Waterfront South district - have not yet been determined. The Reliable Tire Co. inferno destroyed 23 buildings and displaced more than 16 families.
Regarding Sunday's soap plant fire, a man was briefly taken into custody in the morning for questioning and later released, according to sources.
The blaze started around 5:15 a.m. Sunday with flames scaling a 60-foot-high structure on the property, residents said.
The U.S. Environmental Protection Agency had completed a six-month, $1 million cleanup at the site in March, removing asbestos and more than 400 containers of hazardous, flammable chemicals.
Referring to what had until recently been on the site, Robert Corrales, a spokesman for the Mayor's Office, said of Sunday's fire: "It could have been really bad."
The factory has been abandoned for at least a couple years, residents said.
Firefighters from departments across Camden County were called to the 1600 block of Federal Street, an industrial area along the Cooper River.
By 10 a.m., the fire had been brought under control, though flames could still be seen within the charred remains of the former factory.
No injuries were reported. But the fire knocked out power for most of the day in a surrounding three-block area, affecting about 500 customers, according to Public Service Electric & Gas Co. A spokeswoman said 90 percent of the power was restored by 8:15 p.m. and she expected the rest to be flowing later Sunday night.
One home near the fire was evacuated, but residents in a block of rowhouses along Federal Street were allowed to stay put. As smoke from the soap plant drifted through the neighborhood, one man checked on an elderly relative and put up the roof of his convertible.
On the other side of the factory, Louis Santiago, 50, sat on his front steps, watching the firefighters work in the distance.
"I woke up around 8 from the noise. The smoke was so heavy, I was worried for a minute it might spread over here," Santiago said. "The fires, they're all in these warehouse areas. It's weird."
Contact staff writer James Osborne at 856-779-3876 or firstname.lastname@example.org.
Inquirer staff writers Darran Simon and Claudia Vargas contributed to this article.