United Airlines flights were halted nationwide on Tuesday because of an “equipment outage,” according to the Federal Aviation Administration.

One of several FAA bulletins said that United crews were unable to contact airline dispatchers through normal means. The FAA said the issue was limited to United and its subsidiaries.

United did not immediately respond to a request for more information.

Shares of Chicago-based United Airlines Holdings Inc. fell on news of the ground stop and were down more than 3% in afternoon trading.