A company spokesperson confirmed that Karetnikov was employed for nine months as an entry-level software tester.
Karetnikov's Facebook page indicates that he worked as a Software Development Engineer in Test (SDET), located in Redmond.
Microsoft, the spokesperson said, has reviewed his activities and is confident he did not compromise the company’s software or systems.
Karetnikov had been detained since June 28 on immigration violations, according to The Wall Street Journal, and was sent home on Tuesday without being charged with a crime.
Citing unnamed sources, The Wall Street Journal said that Karetnikov came to the attention of FBI agents last fall in connection with the agency's decade-long investigation into Russian "illegals," as the deep-cover spies have been called.
That investigation concluded last month when the U.S. Department of Justice charged 11 people with covertly gathering intelligence for Russia.
Ten of them pleaded guilty to acting as unregistered agents of a foreign government and were deported last week.
The 11th was detained in Cyprus and subsequently jumped bail.
Karetnikov reportedly acknowledged that he was in the U.S. illegally and agreed to leave voluntarily.
It appears that either the U.S. lacked enough evidence to charge him or that the nation interest was better served by dispensing with legal proceedings.
U.S. officials traded the agents they detained for four Russians imprisoned in Russia on intelligence-related charges last week.
The spy swap is widely seen as a way to defuse international tensions heightened by the arrests and to help strengthen relations between the U.S. and Russia, a goal favored by the leaders of both countries.