WASHINGTON -- U.S. President Donald Trump on Wednesday said a Florida congresswoman "fabricated" an account of the commander in chief telling the widow of a soldier killed in an ambush in Niger that her husband "knew what he signed up for."

Democratic Rep. Frederica Wilson said she was in the car with Myeshia Johnson on Tuesday on the way to Miami International Airport to meet the body of Johnson's husband, Sgt. La David Johnson, when Trump called. Wilson said she and others heard part of the conversation on speakerphone.

When asked by Miami station WPLG if she indeed heard Trump say that she answered: "Yeah, he said that. To me, that is something that you can say in a conversation, but you shouldn't say that to a grieving widow." She added: "That's so insensitive."

But in a Wednesday morning tweet, Trump said Wilson's description of the call was "fabricated."

"Democrat Congresswoman totally fabricated what I said to the wife of a soldier who died in action (and I have proof). Sad!" Trump wrote without specifying what proof he had.

Wilson stood by her account, telling CNN on Wednesday that "the president evidently is lying, because what I said is true." Wilson said she and others in the car with Johnson heard Trump.

Escalating the criticism of the president, Wilson said, "He doesn't even know how to sympathize with people. We're grieving. This is a grieving community. ... It's disgraceful for him to even tweet about this. And as I say, this gentleman has a brain disorder and he needs to be checked out."

La David Johnson was among four servicemen killed in the African nation of Niger earlier this month. They died when militants thought to be affiliated with the Islamic State group ambushed them while they were patrolling in unarmored trucks with Nigerien troops.

Wilson said she did not hear the entire conversation, and Myeshia Johnson told her she couldn't remember everything that was said when asked it about it later.

"When she hung up the phone she looked at me and said, 'He didn't even know his name.' Now that's the worst part," Wilson told CNN.

Trump has been criticized for not reaching out right away to relatives of the four killed in Niger. On Monday, Trump said he'd written letters that had not yet been mailed. His aides said they had been awaiting information before proceeding.