Acting Defense Secretary Patrick Shanahan abruptly resigned on Tuesday — and a new report from the Washington Post sheds some light on why.

Shanahan, who had been tapped by President Donald Trump as a permanent replacement for former Defense Secretary James Mattis, admitted to the Post this week that he had written a defense of his then-17-year-old son Will Shanahan after he brutally beat his own mother with a baseball bat in 2011 and left her “unconscious in a pool of blood, her skull fractured and with internal injuries that required surgery,” the paper reports.

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Despite the gruesome nature of the attack, Shanahan sent a letter to his former brother-in-law claiming that his son had acted in self-defense.

“Use of a baseball bat in self-defense will likely be viewed as an imbalance of force,” Shanahan wrote. “However, Will’s mother harassed him for nearly three hours before the incident.”

Shanahan tells the Post that he deeply regrets writing those words and claims that he did not know the extent of his ex-wife’s injuries at the time.

“Quite frankly it’s difficult to relive that moment and the passage was difficult for me to read. I was wrong to write those three sentences,” he said. “I have never believed Will’s attack on his mother was an act of self-defense or justified. I don’t believe violence is appropriate ever, and certainly never any justification for attacking someone with a baseball bat.”

Read the whole report here.