The letter was organized by Gold Star Mother Karen Meredith from VoteVets.org, an advocacy group that calls itself non-partisan but which has been described in the past as allied to Congressional Democrats. The Center for Responsive Politics says VoteVets.org is fueled "largely by social welfare organizations aligned with Democrats and millions of dollars given by unions."
Over the weekend, Trump questioned why Ghazala Khan stood by quietly as her husband Khizr Khan talked about their son Humayun at the DNC.
In the speech, Khan criticized Trump's policies and statements about Muslims. The real estate magnate "sacrificed nothing and no one," Khan said, and questioned whether the Republican had even read the U.S. Constitution.
The Khans' son, a U.S. Army captain, was killed by a car bomb in 2004 while guarding the gates of his base in Iraq, saving the lives of his fellow soldiers and civilians. He was posthumously awarded a Bronze Star and Purple Heart.
Trump, who has called for Muslims to be barred from entering the country, responded to the speech by saying that maybe Ghazala Khan had "not been allowed to have anything to say."
Trump's comments go "beyond politics," according to the letter. "It is about a sense of decency. That kind decency you mock as 'political correctness.'"
It added: "We feel we must speak out and demand you apologize to the Khans, to all Gold Star families, and to all Anericans for your offensive, and frankly anti-American, comments."
The letter's signatories were 10 families whose loved ones died in Iraq and one that lost a father in Vietnam.
Trump did not immediately respond to Monday's letter. He previously hit back at criticism from the Khan family, saying he had "made a lot of sacrifices ... I work very, very hard. I've created thousands and thousands of jobs, tens of thousands of jobs."
However, he tweeted about Khizr Khan on Monday morning.
Trump also took to Twitter Sunday to defend past statements about Islam, terrorism and his record on the war in Iraq.
Trump's comments have sparked a firestorm on social media and forced some GOP grandees to criticize the Republican nominee.
His own running mate, Indiana Gov. Mike Pence, issued a statement Sunday saying he and Trump "believe that Capt. Humayun Khan is an American hero and his family, like all Gold Star families, should be cherished by every American."
Other Republican leaders have also weighed in.
"'Unacceptable' doesn't even begin to describe it," said Sen. Lindsey Graham of South Carolina, who unsuccessfully opposed Trump for the Republican nomination.
"This is going to a place where we've never gone before, to push back against the families of the fallen," he said in a statement. "There used to be some things that were sacred in American politics — that you don't do — like criticizing the parents of a fallen soldier even if they criticize you. "
Ghazala Khan also addressed Trump's comments in an opinion piece in the Washington Post on Sunday, writing: "Here is my answer to Donald Trump: Because without saying a thing, all the world, all America, felt my pain. I am a Gold Star mother. Whoever saw me felt me in their heart."
She added: "Donald Trump said I had nothing to say. I do. My son Humayun Khan, an Army captain, died 12 years ago in Iraq. He loved America, where we moved when he was 2 years old."
The emotional piece recounts how she and her husband worried about their middle son's safety when he was called to fight in Iraq.
"We asked if there was some way he could not go, because he had already done his service. He said it was his duty," she wrote.
She also addressed his comments about Muslims and sacrifice, stating: "When Donald Trump is talking about Islam, he is ignorant."
Ghazala Khan added: "Donald Trump said he has made a lot of sacrifices. He doesn't know what the word sacrifice means."
On Friday, she explained to MSNBC's Lawrence O'Donnell that she was anxious during her husband's speech, knowing her son's photo would appear behind her.
"It was very nervous, because I cannot see my son's picture and I cannot even come in the room where his pictures are, and that's why when I saw the picture on my back, I couldn't take it. And I controlled myself at that time, so it is very hard," she said.
Khizr Khan told O'Donnell that he could not have appeared at the DNC without his wife's close support.
Speaking to NBC's TODAY on Monday, Khan said he was also grateful for an "outpouring" of support he had received in the wake of his speech. He had received many emails "full of assurances that I am right — that we are right."
On Sunday, Khan told "Meet the Press" that "we have a candidate without a moral compass, without empathy for its citizens."
"We don't take these values lightly," he said. "We are testament to the goodness of this country. We experience the goodness of this country every day."
He also responded to a Trump campaign statement saying the candidate he believes Capt. Khan is "a hero to our country."
While he appreciated the clarification, Khan added that "it sounds so disingenuous because of his policies, because of his rhetoric of hatred, of derision, of dividing us. And that is why I implored him to read the Constitution."