United passenger David Dao is pulling out the heavy artillery Thursday as his lawyers and daughter get in front of reporters, and there's a likelihood they will announce a lawsuit against United and the city of Chicago.

Dao's daughter will speak to reporters at 10 AM CT, along with Dao's lawyers. One lawyer is especially significant -- Tom Demetrio, a renowned aviation lawyer who has handled scores of personal injury cases related to airline mishaps.

Demetrio -- the former Prez of the Chicago Bar Ass'n -- filed legal docs Wednesday, asking a judge to order United to retain all videos, cockpit voice recordings, passenger and crew lists, incident reports, police documents related to the incident, and personnel files for the officers involved in removing Dao.

The documents are a prelude to a lawsuit, and it's virtually certain the 2 law firms Dao hired were retained to go after United and the city, which operates the Chicago Dept. of Aviation.

We're told a lawsuit is certain, unless United and the City of Chicago ante up a big check, but that's unlikely.