The Pentagon was tipped off in 2011 by a longtime Army contractor that Russian computer programmers were helping to write computer software for sensitive U.S. military communications systems, setting in motion a four-year federal investigation that ended this week with a multimillion-dollar fine against two firms involved in the work.

The contractor, John C. Kingsley, said in court documents filed in the case that he discovered the Russians’ role after he was appointed to run one of the firms in 2010. He said the software they wrote had made it possible for the Pentagon’s communications systems to be infected with viruses.

Greed drove the contractor to employ the Russian programmers, he said in his March 2011 complaint, which was sealed until late last week. He said they worked for one-third the rate that American programmers with the requisite security clearances could command. His accusations were denied by the firms that did the programming work.