ABC News obtained video of the entire attack at the federal building, where 43-year-old Andrew Arrabaca is allegedly depicted lighting fireworks, firing a BB rifle, spilling gasoline on the stairwell and lighting it on fire.
The video shows that the entire attack unfolded in about two minutes and, during that time, no one appears bothered until flames and smoke begin crawling up the facade, at which point a building security guard tackled Arrabaca and federal police rushed in and arrested him.
Video shows man lighting fireworks outside 26 Federal Plaza
Arrabaca, a former Army private, was charged with arson and did not enter a plea.
Magistrate Judge Robyn Tarnofsky denied bail and remanded Arrabaca into custody.
"The crime you're charged with is troubling," Tarnofsky said. "This wasn't something on a Sunday when nobody was in the building."
Federal prosecutor Samantha Fry said Arrabaca began planning Monday's attack in January when he traveled to upstate New York to buy a pellet gun and a BB gun, and went to Pennsylvania to buy fireworks.
Arrabaca had two axes, a hammer, a machete, three knives and several incendiary devices, according to the FBI.
Fry called the attack "calculated, targeted and unacceptably dangerous."
He is due back in court on Aug. 4.
ABC News contributed to this report.
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