President Trump told the widow of one of the soldiers killed in Niger that he "knew what he was getting into," said U.S. Rep. Frederica Wilson (D-Miami), who said she was in the car where the phone call was received.

Myeshia Johnson was on her way to the airport to greet the remains of her husband, Army Sgt. La David Johnson, when she received the call from the commander-in-chief, CBS Miami reports.

"David was a young man from our community who gave his life for our country," Wilson told CBS Miami. "He's a hero. I was in the car when President Trump called. He never said the word hero. He said to the wife, 'Well, I guess he knew what he was getting into.' How insensitive can you be?"

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The slain soldier's mother, Cowanda Jones-Johnson, stood by the congresswoman's account.

"President Trump did disrespect my son and my daughter and also me and my husband," she told the Post.

The body of Army Sgt. La David Johnson, who was killed in Niger on Oct. 4, returns home to Miami on Oct. 17, 2017. CBS Miami

A White House official did not dispute Wilson's characterization of the phone call. The White House official told CBS News Mr. Trump's conversations with "the families of American heroes who have made the ultimate sacrifice are private."

But Mr. Trump denies the account. On Wednesday morning, he took to Twitter to deny Wilson's assertions:

Democrat Congresswoman totally fabricated what I said to the wife of a soldier who died in action (and I have proof). Sad! — Donald J. Trump (@realDonaldTrump) October 18, 2017

During a meeting with members of the Senate Finance Committee at the White House Wednesday morning, he again denied the account.

"Didn't say what that congresswoman said," Mr. Trump said. "Didn't say it at all. She knows it, and she now is not saying it. I did not say what she said."

Mr. Trump said he had a "lovely" conversation with the widow, but declined to say what "proof" he had of the conversation.

Told of the president's tweet, on CNN's "New Day" on Wednesday morning, Wilson responded, "Well, I don't know what kind of proof he could be talking about. I'm not the only person that was in the car. And I have proof too. This man is a sick man. He's cold-hearted and he feels no pity or sympathy for anyone. This is a grieving widow, a grieving widow who is six months pregnant. This is a young woman. She's only 24 years old. She weighs maybe 110 pounds. And she has two other kids, 2 years old and 6 years old. And when she actually hung up the phone and she looked at me and said he didn't even know his name. Now, that's the worst part."

After a service member is killed, the Defense Department speaks to the designated next of kin, and transmits the information to the Secretary of Defense. The Secretary of Defense passes it to the White House Military Office (WHMO), which also must confirm it. The twice-vetted information then goes to the president and his senior staff.

A White House official told CBS News the WHMO got the information on the soldier's family Thursday and finished confirming it Monday. As soon as that was done, the letters were finalized and sent and calls scheduled, the official said. The calls from Mr. Trump to a designated family member were placed Tuesday, the official added.

CBS Miami reports that after it reached out to Wilson a second time, she repeated that the president told Johnson's widow, Myeshia, that her husband knew what he was signing up for when he enlisted, adding "it still hurts." Wilson said Myeshia was livid and "cried forever" after Trump's call.

Johnson was killed Oct. 4th with three other soldiers in Niger. U.S. officials said they believe extremists linked to the Islamic State of Iraq and Syria (ISIS) were responsible for the attack.

The U.S. and Niger forces in a joint patrol were leaving a meeting with tribal leaders and were in trucks when they were ambushed by 40-50 militants in vehicles and on motorcycles.

After being asked if he called the families of the slain soldiers, Mr. Trump suggested his predecessors did not call the families of dead soldiers. After backlash from officials from the Obama and Bush administrations, who confirmed they did in fact make such calls, Mr. Trump suggested former President Obama did not call White House chief of staff John Kelly, whose son was killed in Afghanistan in 2010. While a White House official said Tuesday that Mr. Obama did not call Kelly, White House records show that Kelly and his wife attended a closed-door breakfast with Mr. Obama and the first lady for Gold Star families, CBS News' Margaret Brennan reported Tuesday.

CBS News' Arden Farhi contributed to this report.