By David Shepardson WASHINGTON (Reuters) -The U.S. Federal Aviation Administration is launching a formal investigation into the Boeing 737 MAX 9 after a cabin panel blew off an Alaska Airlines flight last week in mid-air, forcing an emergency landing, the regulator said on Thursday. The FAA on Saturday grounded 171 Boeing MAX jets with the same panel pending safety inspections. Most are operated by U.S. carriers Alaska Airlines and United Airlines. The incident was the latest in a series of events that have shaken confidence in the aircraft manufacturer. Talks between Boeing, the FAA and airli…