The Chicago Fire Department says the incident appeared to be accidental and that there is no evidence of any criminal activity.
Officials say a United Airlines employee suffered burns on his arm. By late Tuesday afternoon, the incident seemed to be causing minimal disruptions of outbound flights from O'Hare's Terminal 1, but earlier in the day, several arriving United flights were left sitting on the tarmac.
The incident happened around 10:30 a.m., minutes after the United flight landed, triggering a massive response from police and fire crews.
United passenger Walter Reinthaler had just arrived on another plane.
"It was a little scary to pull up and see what was going on," said Reinthaler. "There were probably six or seven big fire trucks, looked like a mobile command unit. There were probably 20 or 25 police cars out there on the tarmac."
The Chicago Department of Aviation says luggage was being unloaded in a ramp area when an industrial battery packed in a checked case accidentally discharged.
The owner of the luggage, who may be an engineer, was questioned by authorities, but everything checked out.
"The biggest thing here is that no criminal activity. The public is not nor was in any danger," said Chicago Department of Aviation Commissioner Rosemarie Andolino. "It was an accidental issue. It's an isolated issue. It's being dealt with and business as usual."
Officials say the burned United worker was taken to Resurrection Hospital and was seen leaving a couple hours later with his arm in a sling.
At O'Hare, several arriving flights were delayed on the tarmac as officials scrambled to assign them to open gates.
"When we landed we were told our gate was full and we would have to circle O'Hare on the ground. They eventually found a spot for us to just park and sit," said United passenger Chris Kennelly.
Janice Vosburgh was on a United flight that arrived from Buffalo and ended up waiting on the tarmac for an hour and a half.
"We couldn't go there, so they put us to another gate, and they couldn't get in there, and then we tried a third gate, and they said the plane was too big for that gate. So then they just parked us out in the back," Vosburgh said.
"We were just told our gate was full and it was part of the hazards of arriving early at a busy airport," said Kennelly. "We just thought we were sitting there waiting to get onto the new gate."
United has not said exactly how many flights were affected, but around midday it appeared that at least a dozen or more arriving flights had to be switched to other gates.