NEW YORK — Democratic presidential candidate Hillary Clinton fell ill during a memorial service marking the 15th anniversary of the Sept. 11, 2001, terrorist attacks, leaving abruptly and inserting new speculation about her health into a presidential campaign in which Republican Donald Trump has called her weak and unfit.

Video of Clinton’s hurried departure from the Ground Zero memorial showed her buckling and stumbling as she got into her van. Clinton’s campaign issued a statement from her doctor later Sunday revealing that she had been diagnosed with pneumonia two days earlier.

The video, circulated on Twitter, appeared to show Clinton, 68, flanked by Secret Service agents, leaning against a security bollard while agents prepare to help her into a black van. As she steps forward, Clinton can be seen falling as agents help lift her into the van.

“Secretary Clinton has been experiencing a cough related to allergies,” Dr. Lisa R. Bardack said in the statement. “On Friday, during follow up evaluation of her prolonged cough, she was diagnosed with pneumonia. She was put on antibiotics, and advised to rest and modify her schedule. While at this morning’s event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely.”

A planned trip to California and Nevada early this week is now under review, campaign officials said.

Campaign spokesman Nick Merrill said Clinton left the ceremony early and retreated to her daughter Chelsea’s apartment in the Gramercy neighborhood of lower Manhattan.

Clinton was not seen for more than two hours, after which she emerged from Chelsea Clinton’s apartment building, walking normally, smiling and waving.

The incident quickly renewed attention on Clinton’s health. Trump has repeatedly questioned her well-being, saying that she doesn’t have the “strength” or “stamina” for the presidency and accusing her of being “exhausted” and sleeping too much.

A coughing episode on Labor Day prompted a fresh round of questions about Clinton’s health. During a speech at a festival in Cleveland, Clinton started coughing repeatedly at the outset of her remarks, took several sips of water and a lozenge and continued to sound hoarse as she spoke. Later that day, Clinton told reporters her condition was caused by “seasonal allergies.”

An initial campaign statement about Sunday’s illness did not mention the pneumonia diagnosis from two days prior, adding to public speculation that the campaign was hiding something. Clinton has followed an intensely busy schedule in recent days, and had appeared healthy when she convened a meeting of national security experts Friday in New York and spoke at a fundraising party that night. It was at that fundraiser where Clinton ignited a contrversy by claiming that “half” of Trump’s supporters are in a “basket of deplorables.”

Neither Trump, who is 70, nor his aides responded to requests for comment Sunday; nor did he weigh in on Twitter or in television interviews, as he is known to do. But attacks from him and his allies, including former New York Mayor Rudy Giuliani, have intensified in the past month as unverified and often debunked theories about Clinton’s health have floated around the Internet. And Sunday’s incident prompted an avalanche of speculation on social media from other Clinton critics.

According to two Trump advisers, Trump decided to stay away from seizing on the Sunday incident, with the aim of bringing Clinton’s “deplorables” comments – for which she has expressed regret – back to the forefront of the campaign this week and avoiding any chance for Clinton to say that Trump was overstepping on the health front.

If confronted by reporters about Clinton’s health, Trump plans to say he hopes she gets well and may raise questions about the campaign’s transparency Sunday, they said.

Both Clinton and Trump had planned to take the day off from formal campaigning in observance of the Sept. 11 anniversary. Trump attended the same memorial service at Ground Zero.

Rep. Peter King, R-N.Y., a Long Island congressman who stood with Trump’s group at the ceremony, said he spoke with Trump shortly after Clinton departed.

“It was actually Trump who told me what was going on,” King said. “He leaned over and told me that ‘Hillary wasn’t feeling well.’ I said, ‘Really?’ That was all he said.”

A former Secret Service agent said that the detail’s movements show they had not planned for her to leave that early and had to make up some rushed security plans on the fly. Clinton’s van was not in place when she arrived at the curb and her detail leader, normally in charge of sticking by her side at all times, had to leave her momentarily to open the door of her van.

A small group of reporters traveling with Clinton were left behind, and not immediately informed that she had left.

Clinton had arrived at the memorial site at 8:18 a.m. Reporters traveling with Clinton became aware about 9:36 a.m. that she was no longer standing where she had been, near several Democratic officials. By 9:48 a.m., her campaign confirmed that Clinton had left the viewing area but offered no more details until about 11 a.m.

Later, shortly before noon, as Clinton exited her daughter’s apartment on East 26th Street, she hugged a young girl and posed for a picture, waved and briefly answered questions shouted by waiting reporters before she departed in her motorcade.

“I’m feeling great, it’s a beautiful day in New York,” Clinton said.

Just before noon, it was 82 degrees and humid at Ground Zero, though it was probably a bit cooler when Clinton had left two hours earlier.

“It was pretty hot out there, but she seemed fine to me, and left on her own accord,” Sen. Chuck Schumer, D-N.Y., who also attended the ceremony, said in an emailed statement.