U.S. regulators on Monday were still investigating the massive salmonella outbreak that sparked a recall of more than a half billion eggs, while lawmakers launched their own probes and consumer advocates urged passage of a law to give food regulators more power.

The U.S. Food and Drug Administration said it still had not determined the cause of the outbreak but had found no evidence of a problem beyond two large Iowa egg producers. Wright County Egg and Hillandale Farms recalled 550 million eggs last week after they were linked to illnesses around the country.

FDA Commissioner Margaret Hamburg called on Congress to pass the FDA Food Safety Modernization Act which would expand FDA's authority over recalls and tracing tainted food back to its sources.

It has been a long-standing gap in FDA's authority, Hamburg said, noting that the recalls were voluntary because the FDA cannot compel product recalls.

The egg recalls came weeks after a new FDA rule took effect requiring large-scale producers to practice better safety and to test in the poultry house for salmonella bacteria.

Had the rule existed earlier, it may have sent the signals of possible contamination soon enough to prevent the spread, Hamburg said, which would have eliminated the need for recalls.

We believe that had these new rules been in place at an earlier time, it would have very likely enabled us to identify the problems on this farm before this kind of outbreak occurred, she told reporters.

The U.S. advocacy group Consumers Union said the country needs stricter food safety laws.

Over the last several years we have seen illnesses and deaths from eating cookie dough, peanut butter and spinach, Consumers Union said in a statement. The pending Senate food safety bill would put FDA in the business of preventing outbreaks, rather than trying to deal with them after people get sick.

U.S. lawmakers sought documents from the FDA and the farms to launch their own probes.

Leaders of the House Energy and Commerce Committee, Representatives Henry Waxman and Bart Stupak, wrote to the farm administrators, seeking details of salmonella contamination.

Chairwoman of the House FDA oversight panel Rosa DeLauro questioned FDA's regulation of Wright County Egg farm.

The extent of the outbreak, combined with the poor regulatory compliance record of this egg producer, leads to questions about what could have been done to prevent it, DeLauro wrote in a letter to the federal regulators.

The salmonella bacteria can cause fever, diarrhea, vomiting and abdominal pain and sometimes more serious illness or death. There have been almost 2,000 cases of salmonella-related illnesses around the country since May 1, hundreds of which have been linked to eggs from Hillandale Farms and Wright County Eggs.

We don't have any evidence that other farms are involved in this outbreak, Hamburg told reporters. However, this is a continuous investigation and we will obviously follow the evidence wherever it takes us.

FDA said it may publish the first findings of its probe later this week.

Common sources of salmonella contamination are rodents, infected chicks, tainted seed or lax safety practices at pullet-rearing facilities.

FDA has eliminated chicks as a possible source because they came from a certified salmonella-free hatchery in Minnesota, said Jeff Farrar, FDA's associate commissioner for food protection.

The recalled eggs were sold under the brand names Sunny Farms, Hillandale Farms, Sunny Meadow, Wholesome Farms and West Creek, Lucerne, Albertson, Mountain Dairy, Ralph's, Boomsma's, Sunshine, Hillandale, Trafficanda, Farm Fresh, Shoreland, Lund, Dutch Farms, Kemps, James Farms, Glenview, Pacific Coast, Alta Dena Dairy, Driftwood Dairy, Hidden Villa Ranch, Challenge Dairy, and Country Eggs.

Kroger Co and Supervalu Inc said several of their California stores have pulled eggs from shelves, including Ralphs, Food 4 Less and Albertsons.