The FBI and Defense Criminal Investigative Service are leading the inquiry. Representatives for both agencies declined comment.
Vergez, who retired from the military in November 2012, and Borisov did not return phone calls seeking comment for this story.
It's unclear when the two men met.
Vergez, 48, spent 25 years in uniform.
He graduated in 1987 from Norwich University, a private military school in Vermont, and earned a master's degree from the Florida Institute of Technology. In the Army, Vergez flew Apache and Cobra attack helicopters. He deployed to Albania in 1999 and later served a tour in Iraq, his service record states. In 2001, Vergez was assigned to the Army command in Hunstville that manages the service's aviation budget.
Borisov, 57, served in the Soviet military for 10 years and launched his aviation companies in the early 1990s. AviaBaltika is based in Kaunas, Lithuania. Saint Petersburg Aircraft Repair Company, better known as SPARC, is headquartered in Russia.
In Lithuania, Borisov is well known for his flamboyant lifestyle and a scandal that led to the impeachment in 2004 of Lithuania's president, Rolandas Paksas. Borisov was the top financial backer of Paksas's election campaign. He contributed $400,000.
The president's troubles began after police alleged that Borisov was in cahoots with the Russian mafia. Paksas's political career fell apart after telephone calls secretly recorded by the state security department became public during the Lithuanian parliament's impeachment proceedings. In one, Borisov is allegedly caught speaking with reputed Russian mob bosses.
Borisov and Paksas denied all the accusations. No charges were filed against Borisov, but the U.S. Embassy in Vilnius, Lithuania's capital, kept a close eye on him. When AviaBaltika applied to the Lithuanian government in 2005 for a license to train Iranian helicopter pilots, Tom Kelly, then the embassy's deputy chief of mission, alerted State Department officials in Washington.