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The Brexit campaign has taken a dark turn.

Labour MP Jo Cox was stabbed and shot today following a constituency meeting in Birstall, West Yorkshire. A witness said the suspect, Thomas Mair, had yelled, “Britain first!” during both the attack and his subsequent arrest by local police.

In response to the attack, campaigning by both the “Remain” and “Leave” campaigns has been suspended out of respect for Cox and her family.

Boris Johnson, London’s former mayor and a leading “Leave” campaigner, said, “We have decided in view of that to suspend all campaigning.” Prime Minister David Cameron, fighting to keep Britain in the EU, tweeted that he would not attend a campaign stop in Gibraltar following the shooting.

Cox was an advocate of staying in the EU. It remains unclear if Mair was referring to the far-right political party known as Britain First, which quickly distanced itself from the incident. “This is absolutely not the kind of behavior we would condone,” said party deputy Jayda Fransen.