For week upon week, Coatesville-area residents wondered who was doing this to them.

Who was lighting the fires that made national news, left scores of people homeless, caused at least $3.5 million in damage, and left Coatesville afraid to sleep? Was it somebody they saw on the street, somebody who stood beside them in the convenience store?

Authorities said yesterday that at least nine of the 24 fires set since Jan. 1 had been the work of a 19-year-old tech student from Downingtown - nine miles away. The suspect had no reported connection to the old steel town, 35 miles west of Philadelphia, that became tarnished and on edge.

Police said Roger Leon Barlow Jr., who received a diploma from the Downingtown Area School District last year, had admitted starting the nine fires, which included the spectacular Jan. 24 blaze that spread across the roofs of 15 rowhouses on Fleetwood Street.

Police did not account for the rest of the fires but said the investigation was not complete. In fact, authorities took a second man into custody last night.

Special Agent John Hageman of the Bureau of Alcohol, Tobacco, Firearms and Explosives said in a statement late last night that Mark Gilliam, 20, of the 1600 block of Suzanne Drive, West Chester, was charged in connection with the Jan. 25, 2009, attempted arson of the Happy Days Family Bistro, 3470 E. Lincoln Highway in Thorndale, about four miles east of Coatesville.

Gilliam, a volunteer firefighter in West Bradford Township who is single and lives with his mother, was arrested at his residence without incident last night, and a search warrant was executed.

He was expected to have an initial appearance today in U.S. District Court in Philadelphia.

A neighbor who witnessed the arrest said agents and police confiscated a computer and other materials from the home.

Finally, authorities indicated that they have very good reason to believe that Gilliam and Barlow know each other.

Earlier yesterday, in announcing the arrest of Barlow, Coatesville Police Chief William Matthews said at a City Hall news conference: "Hopefully, our residents can get a little sleep tonight."

Barlow, described in a court document as 6-foot-3 and 210 pounds, was charged with arson, aggravated assault, and related offenses. He was sent to the Chester County jail to await a hearing Monday, with bail set at $9 million.

"Establishing a clear motive is not possible," said Joseph W. Carroll, the Chester County district attorney.

He said a local, state, and federal arson task force had come up with no evidence that the fires were gang-related, part of any hate crime, or aimed at any particular target.

Barlow's mother, Lorraine, said in an interview yesterday she could not believe her son was responsible for the fires.

She complained that police had seemed to try to trick her son into a confession when they showed up Wednesday night at the family's home in the Downinghouse Apartments. She said they first had asked him about a car chase involving a friend of his.

Lorraine Barlow, 39, spoke while parked in her car on Pennsylvania Avenue in front of the second-floor apartment she shares with her son, husband, and 14-year-old daughter. Units rent for $800 a month, the apartment house manager said.

She said her son had a slight mental handicap and had spent most of his school years in special education at the Downingtown branch of the Devereux Day School.

"I've had my son in counseling therapy ever since he was a little boy," she said.

She said Barlow wanted to be a diesel technician and was enrolled in a training program at the Universal Technical Institute in Exton. She had a paper from the school showing that he had a 95 percent grade average and a 98 percent attendance record.

"He's in school at night, and then he comes home and eats and goes to bed," she said.

She called him a good kid.

"When he was young, he got into trouble, but he never went to prison for it," she said.

John Reed, 22, the apartment house manager, said of Barlow: "He was a little weird, but [the charges] are just a complete shock to me."

Reed's girlfriend, Lia Miller, added: "He seemed a little different, but that was just a complete shock to me."

Many of the Coatesville-area fires started with someone touching a flame to a trash can on a back porch or a sofa out front. Some of the fires were put out without much damage. But in some cases, as on Fleetwood, the fires really took off.

Four Coatesville fires were set overnight Jan. 1-2, but Barlow is charged with just two of them - one that blackened the porch of a single-family house at 205 W. Diamond St., in the West End, and one that engulfed a back porch at 128 N. Fifth Ave.

The next blaze attributed to Barlow was four days later on the Oak Street hill at the city's southern end. The fire seemed to explode from the windows of a wood-frame twin house. A neighbor this week recalled seeing three men burst from the back door as flames gutted the property.

Six days after that, in another fire attributed to Barlow, a mother and three children narrowly escaped a blaze that destroyed the back room of their home at 15 N. Church St.

On and on the fires came. A twin on West Lincoln Highway was hit. So was the 300 block of Fleetwood Street. And a 100-year-old house in a sleepy block on Chester Avenue.

The fires seemed to have no particular target. The victims were black, white, Latino. The neighborhoods were poor, working-class, middle-class.

Mark Potter, special agent in charge of the Philadelphia office of the federal Bureau of Alcohol, Tobacco, Firearms, and Explosives, said the fires had "produced many sleepless nights" for everyone in town - both residents and police officers.

"For residents, it was out of fear," he said. "For law enforcement, it was out of determination."

Potter attributed the arrest to "good, old-fashioned police work." At this point, he said, no one is eligible for any of the $20,000 in reward money.

Still unsolved are 15 fires this year.

At least 26 arsons were reported last year. Although three suspects were arrested in December - one in a fire that killed an 83-year-old survivor of a Nazi work camp - the blazes continued.

Charles Thomas, whose home was badly damaged in the Fleetwood Street fire, said last night: "I'm very happy. I can actually sleep now.

"I would want to know why he wanted to do this to somebody," Thomas said, gesturing to the burned-out block. "Look at all he hurt."

Charred pieces of wood and burned household items littered front lawns. Houses were still set off with police tape, and most windows were boarded up. At one abandoned house, a small pile of Meow Mix was left on the porch, an empty plastic dish nearby.

Several of the fires were in neighboring townships.

Maureen Dailey, 54, of East Fallowfield Township, said she and her family had been "up all night, worried," when one of the fires - not attributed to Barlow - lit up the sky "like a display case."

She, too, felt relief after the arrest.

"I told my son he could put the fire extinguisher away he had next to his bed," she said.