(CNN) The US Office of Special Counsel has opened a case file on whether Interior Secretary Ryan Zinke violated the Hatch Act when he tweeted a photo of himself wearing socks featuring President Donald Trump's "Make America Great Again" campaign slogan.

The internal federal watchdog agency opened the file in response to a complaint from the government watchdog Campaign for Accountability.

"Breaking in new socks on a hike with the governors today," the official @SecretaryZinke account tweeted on June 26 with a photo of Zinke's "Make America Great Again" socks, according to the complaint.

That tweet was later removed from the account and replaced with an apology tweet -- which also no longer appears on the account.

"Earlier I tweeted a pic of my new socks not realizing it had what could be viewed as a political slogan. I've deleted it and apologize for the mistake. I remain excited about all the incredible policy work POTUS is doing," Zinke wrote in the tweet, according to the complaint, accompanied by the same picture with the "Make America Great Again" text blocked out.

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