Federal investigators concluded that former Interior Secretary Ryan Zinke Ryan Keith ZinkeTrump extends Florida offshore drilling pause, expands it to Georgia, South Carolina Conspicuous by their absence from the Republican Convention Trump flails as audience dwindles and ratings plummet MORE violated the Hatch Act when he tweeted out a picture of himself wearing socks with Trump’s “Make America Great Again” campaign slogan, according to a December letter made public on Thursday.

The Hatch Act forbids most federal employees from using their position to promote partisan politics.

The U.S. Office of Special Counsel, which is tasked with investigating possible Hatch Act violations, wrote in a December letter that Zinke broke the law when he posted the picture of his socks from his government account and wore them to a government event.

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“Secretary Zinke engaged in political activity when he wore the above-referenced socks,” Ana Galindo-Marrone, chief of the Hatch Act Unit at the Office of Special Counsel, wrote in a Dec. 20 letter to the watchdog group Campaign for Accountability. The letter was first obtained by The Washington Post.

“Because Secretary Zinke wore these socks to an official event and also authorized their display on his official Twitter account, he violated the Hatch Act’s prohibition against using his official position to influence an election.”

The Office of Special Counsel confirmed the authenticity of the letter to The Hill.

The former secretary avoided repercussions since he deleted the tweet shortly after learning it could have violated the Hatch Act.

"We do not believe that his violation was willful,” Galindo-Marrone wrote.

Punishments for a Hatch Act violation, which are generally decided by the president, could include a $1,000 civil penalty, a reduction in grade or removal from office.

Zinke received a warning letter on Dec. 20, according to the Post, which came after he submitted his resignation letter but before he officially left his office. He was in the midst of multiple ethics probes when he resigned.

Several other Trump administration officials including White House counselor Kellyanne Conway Kellyanne Elizabeth ConwaySpecial counsel investigating DeVos for potential Hatch Act violation: report George and Kellyanne Conway honor Ginsburg Trump carries on with rally, unaware of Ginsburg's death MORE, principal deputy press secretary Raj Shah and deputy director of communications Jessica Ditto have received warning letters for promoting pro-Trump political messages.

“If you’re trying to silence me through the Hatch Act, it’s not going to work,” Conway said last month after she was scrutinized over comments she made about former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE’s presidential campaign. “Let me know when the jail sentence starts.”

Hatch Act violations are not unique to the Trump White House, though they have risen since he took office.

Presidential hopeful Julián Castro, who served as the Housing and Urban Development secretary under President Obama, was found to have violated the law in 2016 when he praised Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE during an interview.

Kathleen Sebelius Kathleen SebeliusSpecial counsel investigating DeVos for potential Hatch Act violation: report The Hill's Coronavirus Report: Mike Roman says 3M on track to deliver 2 billion respirators globally and 1 billion in US by end of year; US, Pfizer agree to 100M doses of COVID-19 vaccine that will be free to Americans The Hill's Coronavirus Report: Former HHS Secretary Sebelius gives Trump administration a D in handling pandemic; Oxford, AstraZeneca report positive dual immunity results from early vaccine trial MORE, the former secretary of Health and Human Services, reimbursed her travel to North Carolina after she weighed in on the state’s gubernatorial race during an official speech there.