A banner was quickly confiscated during Wednesday night's Red Sox game against the Oakland A's.
Four fans seated above the Green Monster dropped down a sign during the fourth inning.
The black sign with white letters read, "Racism is as American as baseball."
Security reeled the banner back within minutes and escorted the fans from their seats, according to CSNNE.
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Red Sox officials confirmed the incident through a statement:
"During the fourth inning of tonight’s game, four fans unfurled a banner over the left field wall in violation of the club’s policy prohibiting signs of any kind to be hung or affixed to the ballpark. The individuals involved were escorted out of Fenway Park," said Zineb Curran, Senior Director of Corporate Communications.
A short time later, a group called Boston Antifa claimed via Twitter to be responsible for the stunt.
Fans at Wednesday night's game were quick to react to the stunt.
"Baseball represents America and racism does not. It's not OK," said one fan.
"I think it is going to end up being another one of those things that makes Boston look bad," said another man. "I don't really know what they meant by it. I guess it is up for interpretation."
This is the second time this year a controversial banner has been unfurled over the Green Monster, according to CSNNE. In June, a banner with a political message regarding the Middle East was also removed.
The Red Sox have instituted a no-tolerance policy after Orioles outfielder Adam Jones, who is black, said he heard racist remarks during a game earlier this season.