Special counsel Robert Mueller briefly did legal work for a consulting company involved in an energy and arms project pushed by President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE's former national security adviser Michael Flynn, who is a subject in the federal Russia probe, CNN reports.

Mueller did "less than half an hour" on litigation work for the entity, IronBridge, during his time at the private law firm WilmerHale, a spokesman told CNN. The work came before Mueller was appointed special counsel in May to oversee the Russia probe.

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IronBridge was among several consulting firms developing a plan for the Middle East that would involve working with Saudi Arabia and Russia's state-run nuclear energy agency to build nuclear energy plants in the country, according to CNN.

The Russian state-run company was sanctioned by the Treasury Department in 2015 for violating federal law banning weapons sales to countries including Iran and Syria, CNN noted.

Mueller's work for the company was not included in his ethics disclosure because it did not meet the guidelines that would have necessitated disclosure, his spokesman told CNN.

"The reporting requirement for public financial disclosure reports requires that you report any source that paid more than $5,000 for your personal services in any calendar year," the spokesman said. "In June 2016 while at Wilmer Hale, Mr. Mueller spent less than half an hour on an IronBridge litigation matter that was handled by another partner. That amount did not meet the reporting requirement."

Reps. Elijah Cummings (D-Md.) and Eliot Engel (D-N.Y.) sent a letter this week to Flynn's former business partners asking for information on a business trip to the Middle East. The Democrats said Flynn appeared to violate the law by failing to disclose the trip.

Mueller has impaneled a grand jury and assembled a group of former FBI agents and leading legal experts for the investigation into the Trump campaign's ties to Russia amid Moscow's election interference.