He appeared to suggest gun violece against Hillary Clinton
A US Secret Service official has spoken to the Donald Trump campaign over his Second Amendment comments.
"There has been more than one conversation," an official told CNN.
The Secret Service is responsible for the security of the serving US president, along with the two presidential candidates.
It comes after the Republican hopeful appeared to hint at gun supporters taking up arms against his rival, Hillary Clinton.
He was speaking about the next president's power to appoint Supreme Court justices - a key election issue since the death of conservative Justice Antonin Scalia earlier this year, who has not yet been replaced.
He told the crowd at the rally in Wilmington, North Carolina: "If (Mrs Clinton) gets to pick her judges, nothing you can do, folks."
Then he added: "Although the Second Amendment people - maybe there is, I don't know."
The Second Amendment protects the right of Americans to bear arms.
Critics said the Republican presidential candidate had advocated that his Democratic rival or her Supreme Court nominees could be shot.
Mrs Clinton's campaign manager Robby Mook, swiftly responded to Mr Trump's remarks, stating: "This is simple - what Trump is saying is dangerous. A person seeking to be the president of the United States should not suggest violence in any way."
Meanwhile Mrs Clinton has accused Mr Trump of "casual inciting of violence" after his remarks.
At a rally in Des Moines, Iowa, Mrs Clinton said Mr Trump "crossed the line" with his comment.
She told supporters his remark showed he lacks the temperament to be commander-in-chief.
Mrs Clinton said that words matter and "if you are running to be president or you are president of the United States words can have tremendous consequences".
Mr Trump insists he never advocated violence and his words were twisted for political purposes.