Parishioners worshipped Thursday at Chicago's iconic Roman Catholic Holy Name Cathedral, undeterred by an attic fire a day earlier that left holes in the roof and led to a water-damaged sanctuary.

"It's just a building, the church is made of people. We'll be OK," said Carla Kupe-Arion, 28, who has been coming to the cathedral since 2005. "I was saddened, but in the same breath, I praised God that not more damage has happened."

Fire officials, meanwhile, ruled out arson and turned their attention to a possible electrical malfunction in a roof deicing system at the 134-year-old cathedral, Chicago Fire Department spokesman Will Knight said Thursday.

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Knight had few details about the system, but said it prevents ice from building up along the church roof's edge. But authorities had not determined the exact origin of the fire and federal investigators from the Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting.

Kupe-Arion was among dozens of parishioners at Thursday morning Mass in the cathedral's adjacent parish center who used plastic and metal chairs for pews and a concrete floor to kneel. Work crews assessing damage blocked the church's main entrance.

A day earlier, firefighters battled the blaze in frigid temperatures at the Gothic revival-style church for more than two hours. The flames severely damaged the attic and left gaping holes in the roof. The smell of charred wood lingered outside the cathedral's massive bronze doors hours later.

Church officials have not offered an estimate on the full extent of the damage, but Archdiocese Chancellor Jimmy Lago said he expected the cathedral to be closed for months.

"Chicago has always bounced back from fires," Cardinal Francis George said Wednesday, after surveying damage inside the cathedral just blocks from the John Hancock Center in a busy shopping district.

The original Holy Name was destroyed in the Great Chicago Fire of 1871 and the parish rebuilt. The new cathedral opened in 1875.

There was no smoke or fire damage in the sanctuary, fire department spokesman Larry Langford said. Fire officials and Chicago police were investigating to determine a cause.

Holy Name's pastor, the Rev. Daniel Mayall, said the fire set off the sprinkler system, which kept the flames from the cathedral's extensive wood paneling and ornate decorations.

But the water damage to the building's interior was "humongous" and crews had to pump water out of the basement, Lago said. He said sacramental records of marriages and baptisms were kept in a fireproof vault and were believed to be safe.

While no injuries were reported from the blaze, a firefighter suffered a minor back sprain after slipping and falling.

A member of a crew that was working on repairs to the church discovered the fire, which appeared to have been burning for a while, said Chicago Fire Commissioner John W. Brooks.

Restoration and repair work on the building had been ongoing after engineers determined structural weaknesses in the roof caused a 10-pound piece of decorative wood to fall 70 feet from the ceiling last February.

Parishioner Jean Valaitis, who has been coming to Holy Name every morning for two decades, said Thursday that the new damage was frustrating.

"It's terrible. We haven't even finished restoring it and here we go again," said Valaitis, 73. "It's very heartbreaking, but it's good nobody was killed."

According to the church's Web site, a 2006 engineering study found "several critical items" in need of updates, including fire protection in the rectory and replacement of the roof membrane. The church had already replaced its floors and refinished pews.

The last major renovation took place in 1968.

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