CHICAGO (CBS) — Federal investigators have begun sifting through the rubble of a Humboldt Park flea market destroyed by a massive fire earlier this week.

The Chicago Fire Department requested assistance from the Bureau of Alcohol, Tobacco, Firearms, and Explosives on Thursday to help determine what started Tuesday’s blaze at the Buyers Flea Market, at 4545 W. Division St. Twenty ATF arson investigators arrived in Chicago on Friday to join the probe. A local ATF team had been working with local fire investigators since day one.

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The extra-alarm blaze destroyed the huge flea market on Tuesday. It took firefighters several hours to get the fire under control after flames ripped through the two-block long building.

It’s the first time the ATF National Response Team has been called in to assist with this type of investigation in Chicago.

ATF agents and Chicago police and fire officials said they are making no assumptions about the cause of the blaze.

“The Chicago Fire Department, the Police Department, and the ATF will jointly work the scene to determine the cause and origin of the fire. I want to make it perfectly clear that we have no preconceived notions or suspicions as to this fire. We need to examine the evidence to narrow our range of possibilities as to the cause, and we will go where the evidence takes us,” Chicago First Deputy Fire Commissioner Richard Ford II said.

He said the sheer size of the two-block long building requires more manpower to work the scene than the Office of Fire Investigations has available.

“Add to this fact that we have over 100 vehicles on the inside, and collapse that has left a tremendous amount of debris where we believe the fire started, the Chicago Fire Department made the decision to call the National Response Team of the Bureau of Alcohol, Tobacco, and Firearms,” he said.

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The ATF also has brought in a mobile lab which can process evidence on the spot.

“Very frankly, this was a no-brainer to bring those resources in so that we could complete this investigation in a thorough and expeditious manner, and get the job done,” Chicago Police Chief of Detectives Eugene Roy said.

Authorities have not yet determined the origin or cause of the blaze, but it’s believed to have started somewhere in the south end of the building, which houses a two-level parking garage and an auto repair shop.

The fire destroyed dozens of businesses that operate every weekend at the flea market. Cars parked on the upper level of the two-story garage were destroyed when they fell through the deck as it collapsed.

“Our first thing is public safety. We’ve got to make sure we shore up the building first before you move forward with this process,” ATF Special Agent Jeffery Magee said.

Authorities said the investigation would be a slow process. Investigators have been interviewing people who were on site Tuesday.

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The owner, who flew in Thursday, said he plans to rebuild.