Frederica Wilson, a Democratic congresswoman from Florida, said she listened in to part of a group phone call between the president and the grieving family of Sergeant La David Johnson.
"I didn't hear the whole phone call, but I did hear him say, 'I'm sure he knew what he was signing up for, but it still hurts,'" she told CNN.
Trump denied making the comments, tweeting Wednesday: "Democrat Congresswoman totally fabricated what I said to the wife of a soldier who died in action (and I have proof). Sad!"
The Miami native's body was returned home yesterday afternoon, according to the Miami Herald. He is survived by his wife Myeshia Johnson, who is pregnant with the couple's third child.
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After the phone call, Myeshia "was crying, she broke down. And she said 'he didn't even know his name,'" added Wilson, referring to Trump.
Trump had faced criticism for not contacting the families of the soldiers killed in Niger right away.
At a press briefing on Monday, he said he had written them letters and would call soon, while accusing his predecessor Barack Obama of neglecting to call as many grieving families as himself.
The statements sparked outrage from former Obama aides, who said Trump's claims were baseless.