(ASSOCIATED PRESS) -- Two more airport police officers involved in dragging a passenger off a United Express flight at Chicago's O'Hare Airport have been placed on leave.

The announcement Wednesday from the city's Aviation Department comes two days after another officer involved in the Sunday night confrontation was put on leave.

The department said it is continuing its investigation.

Dr. David Dao was forcibly removed from a full flight after refusing to relinquish his seat. On Thursday, the city's aviation commissioner is expected to address a city council committee about the incident.

Video of the incident has been shown around the world. The head of United's parent company has scrambled to contain the damage to the carrier's reputation.

Meanwhile, attorneys for Dao took a step toward legal action Wednesday by filing court papers asking the Cook County court to preserve reports and other material related to the incident.