The Federal Aviation Administration is weighing possible temporary action against United Airlines beyond what was described in a letter the company sent to employees Friday, two sources familiar tell CBS News.

This comes in response to a series of concerning incidents involving United aircraft over the last month that included a wheel coming off a Boeing 777, and a panel flying off an aging Boeing 737.

Among the possible temporary measures discussed include barring United from launching new routes that it has not yet begun selling tickets for. Another being considered would be to allow the carrier to continue taking possession of new aircraft — but pausing its ability to introduce the new planes into revenue service, which refers to commercial flights that carry paying passengers.

A third possibility would be temporarily not allowing United check airmen to certify new captains. Airlines typically do those sign-offs internally.