The letter, which was shared with The Washington Post and says it is not a partisan attack, was sent to Trump Monday via United States Postal Service, FedEx and Uber's courier service. It calls for Trump to apologize.

Organizers said they plan to add many more signatories by opening it up online for broader national participation. The list of signatories includes highly decorated combat veterans including Medal of Honor recipient Dakota Meyer, who was a supporter of Ted Cruz's presidential campaign. Rep. Seth Moulton (D-Mass.), who served in Iraq as a Marine Corps infantry officer, is also on the list.

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"To the military and veterans community, nothing is more sacred or honored than our Gold Star parents," says the letter, referring to parents whose children were killed in the line of duty. "Their personal sacrifice, the loss of a child, is the unimaginable. This week, when you chose to disparage the family of an American soldier who gave his life in combat, you chose to disparage all of us."

The letter marks the latest rebuke to Trump's criticism of Khizr and Ghazala Khan, who spoke out against him at the Democratic National Committee last week. The couple’s son, Army Capt. Humayun Khan, was killed by a car bomber in Iraq in 2004.

The note could be an especially potent argument against Trump, who has cast himself as a champion of the military and veterans and has argued that Democratic nominee Hillary Clinton is ill-suited to be commander in chief.

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Trump has said Khizr Khan had no business criticizing him. And he has suggested that Ghazala Khan may not have been "allowed" to speak at the convention. She has said she stood beside her husband but did not speak because she is still experiencing raw emotions about her son’s death and could not bring herself to talk. She said her religion had nothing to do with it.

"Twelve years after losing her son, she is still unable to see a photo of her son without breaking down," the letter says. "Your statements are unacceptable, especially from someone seeking to serve as Commander in Chief. The Khans’ sacrifice has earned them the right to ask hard questions of those seeking elected office."

The group states that they are "writing as a matter of honor and not as a matter of politics, with no support from any campaign."

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The letter concludes: "As Republican, Democrat, and Independent military combat veterans along with Gold Star families, we ask that you demonstrate the character demanded of the office that you seek and apologize for what you’ve said."

Several veterans who signed the letter said in interviews that Trump's comments touched a personal nerve.

"I think this situation speaks quite well to Trump's blatant lack of fitness for office," said James Waters, a former George W. Bush administration White House aide and Navy SEAL. Waters said he is a Republican who is not voting for Trump.

Zach Iscol, a Marine combat veteran who served in Iraq and is a Democrat, said of Trump: "He bullies a lot of people. You do not bully a Gold Star family."

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Ryan Sparks, a Marine combat veteran who served in Iraq and Afghanistan and is a political independent, said, "this is level of language -- it's unacceptable."

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Novelist and TV writer Peter Blauner said an item he posted on social media responding to Trump helped inspire the message in the letter.

"I posted something to the effect of 'What he says makes perfect sense if you ignore the meaning of the word sacrifice,'" said Blauner.

Below is a copy of the letter: