( CNN ) -- Lab tests found hundreds of cases of salmonella contamination at an Iowa farm in a nearly two-year period before the outbreak that prompted a massive recall of eggs this summer , congressional investigators said Tuesday . Wright County Egg is one of two farms at the center of the massive recall . In a letter to its owner , Austin `` Jack '' DeCoster , leaders of the House Energy and Commerce Committee said tests confirmed 426 cases of salmonella contamination between September 2008 and the past July , and 73 were `` potentially '' positive for the strain of the disease involved in this year 's outbreak . The committee 's Democratic leaders asked DeCoster to explain those findings when he appears at a September 21 hearing . They also called on him to explain why those test reports were n't included in material the company has provided to Congress so far , and demanded that the company produce `` all documents relating to your response to the test results '' by Wednesday . `` Despite the committee 's specific request , your response on September 11 , 2010 , did not include the 73 potentially positive results for Salmonella enteritidis , '' Energy and Commerce Committee Chairman Henry Waxman and investigations subcommittee chairman Bart Stupak wrote . `` It also did not show whether Wright County Egg took appropriate steps to protect the public after receiving the positive test results . '' Wright County Egg and another Iowa producer , Hillandale Farms , were forced to recall about 550 million eggs in August after health officials linked them to the salmonella outbreak , which has so far been blamed for more than 1,500 illnesses across the United States . Wright County Egg and other operations owned by the DeCoster family have come under increased scrutiny since the outbreak . In a written statement responding to the letter , the company said it previously pledged to `` fully respond '' to the committee 's requests for documents . `` At that time , we also indicated that our response was ongoing , '' it said . The company said it was `` terribly disappointed '' to find positive tests for salmonella in its eggs , but said it provided results of those voluntary tests `` as soon as those results were available . '' `` We believe our open communication about our forthright efforts to compile testing data and provide documents as requested is consistent with our ongoing cooperation with this investigation , '' it said . `` We have provided the committee with additional testing documentation today and will continue to do so as we restore past testing records . '' Salmonella occurs naturally in chickens , but Food and Drug Administration inspectors reported in August that neither Wright County nor Hillandale fully followed their plans to prevent Salmonella enteritidis . Federal investigators reported finding salmonella bacteria in chicken feed and in barn and walkway areas at the farms , as well as rodents , piles of manure , uncaged birds and flies too numerous to count , the FDA said .
Congressmen demand answers from Iowa farm 's owner . Reports find 426 cases of salmonella contamination in 2 years . Company has come under close scrutiny since summer outbreak .