A group of families of fallen service members wrote a letter Monday condemning Donald Trump’s “anti-American” comments toward the parents of a slain Muslim U.S. soldier, and demanded an apology from the Republican presidential hopeful.

“Your recent comments regarding the Khan family were repugnant and personally offensive to us,” the group of 11 “Gold Star” families wrote. “We feel we must speak out and demand you apologize to the Khans, to all Gold Star families, and to all Americans for your offensive, and frankly anti-American comments.”

The letter, published Monday on VoteVets.org, comes amid a chorus of criticism in response to Trump’s burgeoning feud with the Khan family, which began after Khizr Khan, the father of slain Army Cpt. Humayun Khan, delivered a forceful condemnation of Trump in a speech at the Democratic National Convention. Trump responded by criticizing his wife, Ghazala Khan, for her silence during his speech and suggesting she wasn’t allowed to speak.

“Ours is a sacrifice you will never know. Ours is a sacrifice we would never want you to know,” the families wrote in the letter.

The group hit Trump for questioning why Khan’s wife didn’t speak at the convention, writing: “When you question a mother’s pain, by implying that her religion, not her grief, kept her from addressing an arena of people, you are attacking us. When you say your job building buildings is akin to our sacrifice, you are attacking our sacrifice.”

And the group rejected the idea that Trump was being unfairly criticized for his remarks out of “political correctness.”

“You are not just attacking us, you are cheapening the sacrifice made by those we lost. You are minimizing the risk our service members make for all of us. This goes beyond politics. It is about a sense of decency. That kind decency you mock as ‘political correctness.'”

Demanding an apology, they finish the letter by saying, “We hope you will hear us.”

The Trump campaign did not immediately respond to a request for comment.