Lawmakers voted Thursday to subpoena nine companies responsible for analyzing the most dangerous food entering the country as part of an investigation that gained more urgency with an outbreak of salmonella from tomatoes.

For months, a House Energy and Commerce subcommittee has investigated the possible circumvention of government import alerts. Foods posing a potential danger can enter the marketplace only after a laboratory has determined that they are safe, according to Food and Drug Administration rules. But investigators have been told that it is a routine practice for private labs to test food until a clean result is obtained.

"This repeated testing is done without FDA knowledge that potentially dangerous food has been imported into this country and has entered commerce," said Rep. Bart Stupak, D-Mich., and chairman of the House subcommittee that authorized the subpoenas.

Stupak said nine of 10 companies declined to submit information voluntarily out of concern that the food import companies that hire them would then sue them for breaching confidentiality agreements. The records sought related to testing of food found not to meet FDA standards for import into the U.S.

After the 10-0 vote to issue the subpoenas, lawmakers heard from an array of witnesses critical of the FDA's strategy for improving food safety. The hearing took place as the agency deals with an outbreak of salmonella that has sickened 167 people in 17 states.

Lawmakers said they were frustrated that the agency had yet to identify the source of the contamination and stressed that they were alarmed about yet another outbreak. Citing previous safety issues with spinach, seafood and cantaloupes, they made clear that some blame for the pattern lies with the FDA.

"You've had time," said Rep. Marsha Blackburn, R-Tenn. "We're still waiting."

The FDA is responsible for ensuring the safety of about 80 percent of the nation's food supply — virtually all foods except for meat, poultry and eggs. It released a food protection plan in November 2007 that provided a blueprint for preventing and responding to food-borne illnesses.

The Government Accountability Office issued a report Thursday that the agency has provided few details on the resources and strategies required to enact the plan. For example, the agency has estimated that it would take five years to fully implement the plan but no timelines for the various strategies described.

"We continue to have concerns about FDA's lack of specificity on the necessary resources and strategies to fully implement the plan," the GAO concluded.

Stupak noted that the administration has recently called on Congress to add $275 million in funding for the FDA in the next fiscal year. About $125 million would go to food safety efforts.

"I strongly applaud this request, but we need to know far more details about how this money will be spent," Stupak said.

The nine companies getting subpoenas were ABC Research Corp. of Gainesville, Fla.; Analytical Food Laboratories Inc. of Grand Prairie, Texas; Central Analytical Laboratories Inc. of Metairie, La.; Certified Laboratories Inc. of Plainview, N.Y.; Michelson Laboratories Inc. of Commerce, Calif.; Microbac Laboratories Inc. of Wexford, Pa.; The National Food Laboratory Inc. of Dublin, Calif.; Northland Laboratories of Northbrook, Ill.; and Strasburger and Siegel Inc. of Hanover, Md.