In a response sent to SecurityFocus on Monday, the Department of Homeland Security said its personnel "take the Committee's questions very seriously and have devoted incalculable man hours to responding to their questions." The federal agency, which is responsible for counterterrorism and helping protect federal systems against cyberattack, declined to comment on the specifics of the incident described in the Congressional letter.

In general, however, the DHS has investigated a number of incidents, DHS spokeswoman Laura Keehner said in a statement sent to SecurityFocus.

"We are aware of, and have responded to, malicious cyber activity directed at the U.S. government over the past few years," Keehner stated. "We remain concerned that this malicious activity is growing more sophisticated and frequent."

The contract held by Unisys ends this year. The Department of Homeland Security declined to renew the contract's option for a final year and, instead, has rolled up many of its information technology contracts into a single bid, Keehner stated.

"Unisys is not prohibited from competition, but previous performance can be a factor in competition for future contracts," she said. "The evaluation has not been completed; therefore a selection has not yet been made."

The FBI has reportedly opened an investigation into the matter, according to media reports.

Unisys's stock fell nearly 4 percent on Monday from its opening price of $7.11. The stock fell another 1 percent on Tuesday morning.

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