A Philadelphia sheriff's deputy was critically injured Thursday when a structural failure involving a pair of employee elevators shattered the morning with a boom that some feared was a bomb and forced evacuation of the Criminal Justice Center in Center City.

According to Mayor Kenney's office, elevator Car 2B, which contained one passenger, rose through the 15th-floor ceiling into the equipment "penthouse," which houses the motors.

That dislodged concrete and other debris, which rained down an adjacent shaft onto the roof of Car 1, which was at the basement level with one passenger aboard.

Deputy Fire Commissioner Jesse Wilson said Sgt. Paul Owens, 50, a 29-year sheriff's deputy who was in Car 2B, suffered significant head and chest injuries and was taken to Hahnemann University Hospital, where he was reported to be in critical condition. Sheriff Jewell William said later at the hospital that Owens also has suffered broken bones in his back.

A court employee in Car 1, which dropped a short distance after it was struck by debris, was taken to a hospital, where she was treated for minor injuries.

About 10:20 a.m., what sounded like an explosion rocked the 17-story building, followed by the sound of debris rattling down the shafts.

The incident triggered fire alarms throughout the building, and employees and others began the trek down 14 floors of stairs to the lobby and then out to the sidewalks on 13th and Filbert Streets.

Sheriff Williams said the explosive sound "was the noise of the malfunction."

Shortly after noon, officials announced that the criminal courthouse would be closed for the rest of the day and on Friday so its elevators could be inspected.

Kenney's office said city records showed that all elevators in the Criminal Justice Center were inspected by the Pennsylvania Department of Labor and Industry on June 14, 2016, and passed inspection.

The remaining six elevators were cleared again Thursday by state inspectors. The Department of Licenses and Inspections will require that the elevator shaft ceiling be repaired and the elevators recertified by the state before that bank of elevators can be reopened.

The labor department is required by law to inspect elevators at least every six months and file reports on the results.

The department, in a statement after the incident Thursday, said it was invoking "an investigation exception" in the state's Right-to-Know law when the Inquirer and Daily News requested copies of the most recent inspection reports for the center's elevators. That exception allows an agency to withhold public records during an investigation.

"The rationale is essentially to not hamper investigations/inspections and to protect the privacy of [those] being investigated," the department's statement read. "We cannot comment on an active investigation, which is what this situation is now."

The elevators involved are not used by the general public. They are in the back of the building, behind a key-card security door, and are used by employees, judges, deputies, and jurors, Williams said.

"Thank God there were not more people in the elevators," said Williams.

The Fire Department said the initial call was for civilians trapped in an elevator.

Municipal Court Judge Patrick Dugan said he was in a meeting, planning for hearings in Veterans Court, when he heard a loud explosion.

"I was on the 10th floor," Dugan said while standing outside the center. "I felt the building shake. The first thing you think about when you hear an explosion? You're hoping it's not somebody targeting the building."

Dugan, who served with the Army Reserve in Iraq and Afghanistan, said the incident "sounded like the explosions every day we received, particularly in Mosul."

Lamont Fitzgerald, 26, of West Philadelphia, said he was in a fifth-floor courtroom when emergency lights went on. It was five minutes until everyone was told to evacuate, he said.

The fire alarm disabled all eight public elevators and people were directed to the crowded stairs, he said.

"I didn't move for 15 minutes," Fitzgerald said. He said he thought the evacuation was handled poorly. People in wheelchairs were left at the top of the steps to wait for fire crews, he said.

"They were told there was no immediate danger," he said.