Rep. Mike Coffman, a Colorado Republican leading the hearing, said a report from the Centers for Disease Control and Prevention indicated that five patients may have died from the respiratory infection.
The VA previously had said 29 Legionnaires' cases were recorded at the hospitals since January 2011, and local officials confirmed one death.
Rep. Tim Murphy, R-Pa., said the information raises new questions about whether the outbreak could have been prevented.
Legionnaires' disease most often strikes the elderly and can cause deadly pneumonia. It's caused by bacteria that can be spread through mist or vapor from contaminated water or air conditioning systems.
The VA announced the outbreak Nov. 16 and switched its water treatment systems at two hospitals in Pittsburgh.
In testimony Tuesday, the VA said the VA Pittsburgh Healthcare System and CDC identified 29 cases of veterans with Legionella pneumonia from January 2011 through
November 2012 and five cases were confirmed to have originated at the VA hospitals. Four of those patients recovered, and one died within a month of the diagnosis, and the infection was listed as a contributing cause of death.
Sixteen other infected patients had contact with the hospitals, so they could have contracted the disease there, but the VA said there's no way to be sure; it said the CDC listed those cases as "probable hospital-acquired." Eight cases were contracted outside the hospitals.
VA spokesman Mark Ballesteros said Tuesday he doesn't have any more information than what was provided at the hearing. The CDC declined to release the report.
Coffman also alleged that VA workers were caught falsifying data about water treatment levels in December 2011. But regional VA administrator Michael Moreland said he had no evidence of that, the Pittsburgh Tribune-Review reported.
In 2011, the CDC said that cases of Legionnaires' disease tripled in the last decade. The agency reported 3,522 cases in 2009, the most since Legionnaires' was first identified in 1976 following an outbreak at the Pennsylvania American Legion convention held at the Bellevue-Stratford Hotel in Philadelphia. The agency reported 1,110 cases in 2000, and it said the increase may be partly because of the growing number of old people.