Gilliam, who sources tell Action News was a one-time volunteer firefighter, was arrested at his residence.
RELATED: Read the affidavit against Mark Gilliam.
Agents raided his home in the 1600 block of Suzanne Drive in West Bradford Township and hauled away a number of items said to be potential evidence.
It is expected that Gilliam will have his initial appearance on Friday in U.S. District Court in Philadelphia.
Earlier in the day, police in Coatesville arrested 19-year-old Roger Leon Barlow, Jr., of Downingtown who they say is responsible for a number of deliberately set fires in the area.
Barlow is charged with arson and related charges.
His bail is set at $9 million
RELATED: Press conference on the arrest of Roger Barlow.
Sources tell Action News Gilliam and Barlow recently became friends.
Federal and state arson investigators hauled off a number of items from the apartment that Barlow shared with his mother and sister. Among the items seized, the mother says, was poetry her son had written.
The mother is adamant that her son, who she says is a mentally challenged vo-tech student in Exton is not an arsonist.
Mrs. Barlow earlier told Action News her husband, Roger Barlow, Sr. was a firefighter for the Thorndale fire company, but the company refutes that, saying he is merely a member of their social club.
Roger Sr. himself is astonished that his son has been implicated in the arsons.
Both parents claim their son was wrongly implicated by a 20-year-old her son recently became friends with who had a brush with the law and who they believe is the real arsonist.
However, State Police say Barlow admitted guilt to 9 of the fires, including the Fleetwood Street fire on January 24 and a fire on Chester Avenue on February 3.
RELATED: Roger Barlow, Jr.'s mother spoke to Dann Cuellar in an exclusive interview.
"23 reported arsons in a 37 day period produced many sleepless nights. For citizens it was out of fear, for law enforcement it was out of determination," Mark Potter of the ATF said during a news conference on Thursday.
Carroll calls Barlow a pyromaniac who liked to watch things burn.
At the news conference, Caroll said the arsons do not appear to have been gang related, adding at this point, there is no evidence that the fires were result of hate crimes or that Barlow targeted any specific groups or individuals.
Barlow's MySpace page, which has since been taken down, featured a fire widget that showed flames.
RELATED: Archive of of stories concerning the Coatesville arsons.
The Chester County town of 10,000, set 35 miles west of Philadelphia, has seen 18 cases of arson and 5 others in the surrounding area since the beginning of 2009. There were another 26 intentionally set fires reported in the city last year.
Authorities have tried a number of measures to thwart additional arson attempts. The Chester County Arson Task Force offered a $20,000 reward for information leading to an arrest, and earlier this month authorities distributed about 800 motion-sensitive outdoor lights to roughly 200 Coatesville residents.
In an attempt to narrow their field of suspects, police lowered an already mandated 10:00 p.m. curfew for those under 18 to 8:00 p.m.
Federal and state police assisted local authorities with the investigation.
The first arrest of the day was on state charges, while the second was a federal arrest.
Police will not disclose information about any other possible suspects, but say the investigation still continues.
An arson task force focused on Barlow after police stopped his vehicle during elevated patrols, said Mark Potter, special agent in charge of the Bureau of Alcohol, Tobacco, Firearms and Explosives.
Investigators asked Barlow what he was doing in town. Based on his reaction - and a later glance at his MySpace page, which includes a box showing animated flames - officers knew they wanted to talk to him further, Potter said.
Arsonists are typically motivated by thrill-seeking, revenge or hate, Potter said.
"Roger Barlow's dangerous and risky behavior for the past 48 days, and maybe longer, has now been stopped," he said.
With Barlow in custody, investigators said they will take a new look at the other unsolved arsons and determine if he had a role in them.
The Associated Press contributed to this report.
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