Democratic presidential nominee Hillary Clinton left an event commemorating the September 11, 2001, terror attacks in New York after feeling "overheated," according to a campaign statement.
A video shared widely on social media appeared to show the former secretary of state stumbling and requiring assistance as she entered a van.
"Secretary Clinton attended the September 11th Commemoration Ceremony for just an hour and thirty minutes this morning to pay her respects and greet some of the families of the fallen," said a statement distributed to media by Clinton spokesman Nick Merrill.
"During the ceremony, she felt overheated, so departed to go to her daughter's apartment and is feeling much better."
Emerging later from her daughter Chelsea's apartment, Clinton said: "I'm feeling great, it's a beautiful day in New York," according to the Washington Post.
The incident comes amid recent speculation about the state of Clinton's health by supporters of Republican candidate Donald Trump, which Clinton supporters have dismissed as conspiracy theories.
Neither 68-year-old Clinton nor 70-year-old Trump have released their official medical records.