The ATF offered a reward of up to $5,000 for information leading to the arrest and conviction of those responsible for Sunday's arson, Horace said, urging anyone with information to call a newly created hotline at 1-888-283-3473.
"That investigation is continuing. It is vigorous, and it will be pursued until we find the persons responsible," Faulk said.
The ATF's national team of experts, including electrical engineers, chemists, and other specialists, was sent to Camden this week at the request of local officials. The ATF has not turned its attention to the other fires, Horace said.
"I am not sure that we are going to have the same amount of information from the first two fires that was obtained by this national response team from the ATF in this third fire," Faulk said. "So I don't know what sort of comparisons can be made."
But comparisons will be made if possible, he said.
The city has about 3,000 abandoned buildings, many of which are used frequently by drug dealers, prostitutes, and scrap metal thieves.
Police Chief Scott Thomson said patrols would be increased around some vacant buildings, and he asked residents to "remain ever vigilant."
Sunday's blaze broke out around 5:15 a.m. at the former Concord Chemical building on the 1700 block of Federal Street. One home was evacuated, and one person was taken to a hospital.
The first suspicious fire this month was a 12-alarm blaze June 9 that started in a former tire-distribution center near the city's Parkside section that damaged 23 buildings, 10 of which were occupied. More than half of the buildings were severely damaged, and 16 families lost their homes.
A second, eight-alarm fire occurred June 11, gutting a former garment factory in the Waterfront South neighborhood. Homes near the factory were largely spared.
The three industrial buildings were made of brick and heavy timber, Fire Chief Michael Harper said.
A fourth warehouse fire, a minor one-alarm blaze, broke out Thursday around 8 p.m. in an abandoned building on the 1000 block of Magnolia Avenue in the Parkside section. The cause of that fire remains under investigation.
Contact staff writer Darran Simon at 856-779-3829 or dsimon@phillynews.com.
