Democratic presidential nominee Hillary Clinton left a Sept. 11 memorial service early after feeling "overheated," according to a campaign spokesman, and video of her departure showed her buckling and stumbling as she got into her van. (Thomas Johnson/The Washington Post)

Democratic presidential nominee 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 several Secret Service agents, leaning against a security bollard while agents prepare to assist 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,” Lisa R. Bardack, Clinton’s physician, said Sunday 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 on Monday and Tuesday has been canceled, campaign officials said late Sunday. Clinton had been scheduled to attend several fundraising events across the state, in addition to a major economic speech in Southern California and a taped appearance on the talk show “Ellen.” It remained uncertain whether Clinton would continue with her planned travel to Las Vegas on Wednesday.

Hillary Clinton left a New York memorial service marking the 15th anniversary of the Sept. 11 attacks early after feeling "overheated," according to a campaign spokesman. She emerged from her daughter's New York apartment later and said she feels better. (The Washington Post; Photo: Yana Paskova, The Post)

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 to 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 had 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 she had appeared healthy when she convened a meeting of national security experts Friday afternoon in New York and then spoke at a fundraising party that night. It was at that fundraiser where Clinton ignited a controversy 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 Rudolph Giuliani (R), have intensified in the past month as unverified and often debunked theories about Clinton’s health have floated on 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 that he hopes she gets well and may raise questions about the campaign’s transparency Sunday, they added. The advisers spoke on the condition of anonymity because they were not authorized to discuss campaign deliberations.

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 T. 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 in an interview Sunday. “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 security detail’s movements showed that the agents had not planned for her to leave that early and had to make some rushed security plans on the fly. Clinton’s van was not in place when she arrived at the curb, and her detail leader, who normally sticks 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 was left behind and was not immediately informed that she had departed.

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.

Shortly before noon, as Clinton exited her daughter’s apartment building on East 26th Street, she hugged a young girl and posed for a picture, waved and briefly answered questions shouted by 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, although 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,” said Sen. Charles E. Schumer (D-N.Y.), who also attended the ceremony, in an emailed statement.

Rep. Joseph Crowley (D-N.Y.) said it is unfortunate that this episode will feed into conspiracies about Clinton’s health. He said anyone could have been similarly affected and Clinton tends to be held to her own “demigod” status.

Clinton has been generally healthy, with the exception of clotting in one leg in 1998 and a concussion and associated health problems from a fall in December 2012. But she has been attacked repeatedly by critics and accused of hiding more serious health issues.

Giuliani, a close adviser to Trump who is regularly at his side on the campaign trail, said last month that he thinks Clinton is “tired” and “looks sick.”

“What you’ve got to do is go online,” Giuliani said on “Fox News Sunday” in late August, accusing the media of hiding information about Clinton’s health. “So, go online and put down ‘Hillary Clinton illness,’ take a look at the videos for yourself.”

Trump showed up to the ceremony with no formal notice to reporters who cover him, and there were no arrangements for a media pool. This is not the first time that Trump has avoided having a media pool — last month he traveled without reporters to Louisiana to view flood damage and to Mexico City to meet with the Mexican president.

After attending the ceremony, Trump and Giuliani briefly visited New York City Fire Department Rescue Company 1. An NBC News reporter there asked Trump about Clinton’s “health incident this morning.” Trump shook his head, frowned and said: “I don’t know anything about it.”

If he wins in November, Trump, 70, would become the oldest president ever elected. In December, Trump released a four-paragraph letter signed by physician Harold N. Bornstein of Lenox Hill Hospital in Manhattan that contained few specifics but declared that Trump would “be the healthiest individual ever elected to the presidency.”

The letter pales in comparison to the more than 1,000 pages of medical records released in May 2008 by Sen. John McCain (Ariz.), who was then 71 and went on to become the Republican presidential nominee. The records detailed eight years of care that McCain received while fighting cancer.

Trump’s spokeswoman, Hope Hicks, said in late August that Trump has “no problem” releasing a full medical history, as long as Clinton does the same. Last week, Trump said that he is willing to go first. On Friday, “The Dr. Oz Show” announced that Trump and his eldest daughter, Ivanka Trump, will appear on the show on Sept. 15 to discuss Trump’s “own personal health regimen.”

Clinton’s 2012 episode led to a brief hospitalization for a blood clot in her head. Details on Clinton’s condition were initially hard to come by, but her State Department office eventually provided extensive medical information.

Clinton wore special corrective glasses for months, and her husband, former president Bill Clinton, told an audience in 2014 that it had taken about six months for his wife to recover fully. Clinton has said she was surprised by the illness because she had not experienced anything like it before.

Clinton’s campaign released a memo from her personal physician, Bardack, in July 2015, pronouncing the candidate healthy and suffering no lasting effects from the concussion.

The 2012 concussion caused concern among Clinton friends and supporters who hoped that she would make a second run for the presidency, some of whom predicted correctly that the episode would fuel speculation that Clinton was too frail to be commander in chief.

Her campaign dismisses any suggestion that the candidate is not up to the job, while suggesting that the speculation is an example of a sexist double standard that is not applied to male candidates.

Clinton is the first woman to become a major-party presidential nominee. The health questions come as she tries to make a more personal and direct appeal to voters that focuses on her credentials and background.

As rumors have mounted about Clinton’s health in recent weeks, her running mate, Sen. Tim Kaine (D-Va.), has inserted a short section into his speeches about how much stamina she has.

During a keynote address Saturday night at a Human Rights Campaign dinner, Kaine said: “I can’t imagine the stamina and energy it takes to run this campaign for 18 months,” Kaine added. “This is one determined lady.”

Gearan reported from Washington. Kayla Epstein and Philip Bump in New York and Jenna Johnson, Carol D. Leonnig, Robert Costa and John Wagner in Washington contributed to this report.