A semi crashed under an overpass and people working in buildings next to it said it was loud and scary.
Some thought the nearby federal building was under attack.
Employees at the IRS building said they heard a loud noise and felt the entire building shake.
Angela Harbin and others working in the IRS building rushed to a higher floor after the initial explosion.
"It just rattled the room, so we had to see what was going on," Harbin said. "We see smoke, and we see flames."
Harbin and her coworkers had a front-row seat to an inferno on Interstate 44, right by the Highway 169 bridge.
Next door, at Sherwood Companies, Jackie Cheney counted several explosions.
They thought something had happened to the federal building.
"We work right next to the IRS building, so we didn't know if it was that building, didn't know if it was a plane because planes fly over here a lot," Cheney said.
She says they were all watching the flames and billowing smoke.
"It was scary, but we were all standing there thinking oh my god hope nobody was hurt," Cheney said.
Some drivers made it over the bridge before both 169 and I-44 before first responders shut down lanes in every direction to fight the fire.
The people who saw this, say it's an experience they won't soon forget.
"We were hoping and praying that everyone got out," Harbin said.