(CNN) Democrats are demanding answers from the White House chief of staff and counsel about former aide Rob Porter, who resigned Wednesday amid allegations of domestic abuse.

A group of twelve Democratic senators sent a letter Friday to chief of staff John Kelly and White House counsel Don McGahn asking when they first knew of the domestic abuse allegations against Porter by his ex-wives.

"We recognize that you and the President have tremendous discretion in deciding whom to hire to work in the White House," the letter read. "However, we are troubled by published accounts suggesting that you decided to hire Mr. Porter despite the fact he could not get a security clearance and that you were aware of the specific domestic violence allegations made against him."

The senators also question whether Porter, who served as Trump's staff secretary, disclosed the allegations on his applications for a security clearance or White House employment. They said they wanted to know if Porter was ever denied a security clearance and if he handled classified information with an interim security clearance.

The allegations of domestic abuse against Porter first came to light Tuesday.

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