The head of Major League Baseball has said the organisation is investigating a tweet from an umpire that threatened civil war if US President Donald Trump was impeached.
Rob Drake's now-deleted tweet said he planned to buy an AR-15 rifle. It comes amid an impeachment inquiry in the US House of Representatives.
Mr Drake, 50, has not commented and has deactivated his Twitter account.
He has been a full-time major-league umpire since 2010.
In the tweet, Mr Drake said he planned to buy an assault rifle "because if you impeach MY PRESIDENT this way, YOU WILL HAVE ANOTHER CIVAL (sic) WAR!!! #MAGA2020", according to a copy seen by ESPN.
It reportedly followed another tweet that read: "You can't do an impeachment inquiry from the basement of Capital (sic) Hill without even a vote! What is going on in this country?"
Major League Baseball (MLB) Commissioner Rob Manfred told reporters before a World Series game on Wednesday that the tweet would be looked into.
The president of the Major League Baseball Umpires' Association has declined to comment.
The impeachment inquiry is examining whether Mr Trump abused his office by improperly pressuring Ukraine to investigate his political rival, former US Vice-President Joe Biden.
Mr Trump insists he has done nothing wrong and recently called the inquiry "a lynching", sparking outrage from Democrats.