President Trump told the widow of one of the U.S. service members who was killed in an ambush by Islamic State-linked militants in Niger this month that the soldier knew "what he signed up for, but I guess it still hurts," during a phone call Tuesday, according to a Democratic lawmaker.

Rep. Frederica Wilson, D-Fla., said she overheard the conversation Trump had with Army Sgt. La David Johnson's widow, Myeshia Johnson, as they traveled together to meet Johnson's body in Miami.

Trump apparently challenged Wilson's claim in a tweet Wednesday: "Democrat Congresswoman totally fabricated what I said to the wife of a soldier who died in action (and I have proof). Sad!"

Wilson, in an interview on CNN minutes after Trump's tweet, rejected the president's claim, saying "I have proof, too." Wilson said she, members of Johnson's family and others heard parts of the conversation on speaker phone.

"So the president evidently is lying, because what I said is true," she said. "I have no reason to lie on the president of the United States with a dead soldier in my community."

Myeshia Johnson, who is six months pregnant with the couple's third child, was very emotional after the phone call, Wilson said Tuesday.

“To me that is something that you can say in a conversation, but you shouldn't say that to a grieving widow," Wilson said. "Everyone knows when you go to war you could possibly not come back alive. But you don't remind a grieving widow of that. It's so insensitive."

Wilson said Johnson could not remember everything Trump said, but felt that Trump didn't know her husband's name. Wilson told CNN that Trump needs to learn from President Obama and become "presidential." She said she was "livid" when she heard Trump's comment.

"I asked them to give me the phone because I wanted to speak with him," Wilson told CNN. "And I was going to curse him out."

She said a master sergeant who was in the car declined her request for the phone.

Wilson said she knew Johnson's family before the tragedy and has started a GoFundMe campaign aimed at funding scholarships for Johnson children. The campaign had exceeded $250,000 on Wednesday.

Trump was widely criticized after he defended his slow public response to the deaths of the soldiers, falsely claiming that most of his predecessors never called the families of service members killed in action.

Four soldiers were shot and killed while on a reconnaissance patrol in the west African country on Oct. 3. They were serving in Niger as part of an operation to train local forces to combat the Boko Haram terror group, which has ties to the Islamic State and al-Qaeda.

Wilson said she wants an investigation into the circumstances surrounding the ambush.