Federal investigators are saying that they have recently learned that Topps Meat Company failed to require adequate testing on the raw beef it bought from its domestic suppliers, and it sometimes mixed tested and untested meat in its grinding machines.

The Agriculture Department acknowledged that its safety inspectors, who were in the Topps plant for an hour or two each day, never cited the company for these problems.

Additionally, Topps, like many other beef processors, had bought an increasing amount of meat from overseas. Some types of meat from foreign countries are not required to be tested for contamination.

There have been 15 E. coli O157:H7 recalls in beef this year, eight of which caused illnesses. The USDA now will test meat at large plants about 12 times a year, up from seven.