During that visit, the former Goldman Sachs partner Mnuchin, Linton and a group of other civilians that included Senate Majority Leader Mitch McConnell , R-Ky., visited the gold depository and watched the solar eclipse.

Now CREW is asking the Treasury Department for records about the "authorization for and the costs" of the couple's use of a government plane to travel to Lexington, Kentucky .

A watchdog group suspects Treasury Secretary Steven Mnuchin 's trip to see the government gold in Fort Knox on the taxpayers' dime with his wife, Louise Linton, was actually spurred by a desire to be in the prime viewing area for Monday's total solar eclipse.

CREW also wants "all records" about Mnuchin's "use of a government plane for any purpose since his appointment as Treasury Secretary."

"The requested records would shed light on the justification for Secretary Mnuchin's use of a government plane, rather than a commercial flight, for a trip that seems to have been planned around the solar eclipse and to enable the Secretary to secure a viewpoint in the path of the eclipse's totality," CREW said.

A Treasury spokeswoman provided the following statement to CNBC:

"The Secretary was on official government travel to Kentucky where he and Leader McConnell met with members of the business community to discuss tax reform. Later in the day, the Secretary, Leader McConnell, Governor Bevin, Congressman Guthrie and U.S. Treasurer Carranza visited the United States Bullion Depository at Fort Knox. Secretary Mnuchin is reimbursing the government for Mrs. Linton's travel, as is longstanding policy regarding civilians on military aircraft. The trip was originally planned for earlier in August but was postponed to accommodate the Congressional calendar."

CREW spokesman Jordan Libowitz said concern that Mnuchin was using taxpayer funds for a trip to see the eclipse "would not have been on our radar if it had not gotten the press from the Instagram rant" by his wife, Linton.

"That picture was clearly of a government plane, so that raised questions of what the relationship was of her to the trip, and what she was doing there," Libowitz said. "And it's a little bit out of the ordinary for a secretary of the Treasury to visit Fort Knox."

"I think it's quite possible that" the solar eclipse was "why he and his wife were there," Libowitz said.

Mnuchin is the first Treasury secretary in seven decades to visit Fort Knox, which holds gold bullion owned by the federal government.

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And civilians had not visited the vault in Fort Knox in more than 40 years.

The Washington Post reported that Mnuchin "told a group of Louisville business leaders earlier in the day [Monday] it was important for him to see the gold to attest that 'it is part of our national assets.'"

Linton, a Scottish actress who married Mnuchin earlier this year, ignited an epic social media backlash Monday when she responded to a comment on an Instagram photo that showed her and the Treasury secretary disembarking from a government plane during the Kentucky trip.

Linton had captioned that photo with hashtags highlighting the high-end designers who created the clothes and accessories she was wearing: Tom Ford, Hermes and Valentino.

When Instagram user Jenni Miller commented on the post: "Glad we could pay for your little getaway. #deplorable" — Linton went ballistic with a snarky and scathing post.

"Aw!!! Did you think this was a personal trip?! Adorable! Do you think the US govt paid for our honeymoon or personal travel?! Lololol," Linton wrote.

"Have you given more to the economy than me and my husband? Either as an individual earner in taxes OR in self sacrifice to your country?" Linton wrote. "I'm pretty sure we paid more taxes toward our day 'trip' than you did. Pretty sure the amount we sacrifice per year is a lot more than you'd be willing to sacrifice if the choice was yours."

She added: "You're adorably out of touch."

A day after that tirade, Linton apologized through a publicist for both her initial post and her fulmination against Miller, according to a report on CNN.

"I apologize for my post on social media yesterday as well as my response. It was inappropriate and highly insensitive," Linton reportedly said.