Some Burger King restaurants in seven Washington state counties were found to be serving dangerously undercooked hamburger patties in what health authorities call a “disturbing trend” they want corrected.

In a letter sent September 1 to Burger King’s corporate offices, the Washington State Health Department said, “As we discussed, we are deeply concerned about reports relating to undercooking hamburgers at many Burger Kings in our state.

“Inspection reports from several of our local health jurisdictions show a disturbing trend.”

On July 29, a Tacoma-Pierce County Health Department inspector found that hamburgers at a Puyallup Burger King had been cooked to only 131 degrees. Studies have shown that pathogenic E. coli can survive at 140 degrees. The federal recommendation for ground beef safety is 160 degrees.

After undercooked burgers were then discovered at six of the 13 Burger King locations in Pierce County, health authorities alerted state regulators, who found similar violations at Burger Kings in other counties, including at four outlets in Seattle’s King County.

In the letter to Burger King Corp., Washington public health officials said some of the fast-food company’s workers did not know how to test the temperature of cooked burgers, did not know undercooked burgers should be discarded and assumed a brief warm up in a microwave oven would make the undercooked burgers safe to eat.

Health authorities cited problems with the company’s  “flamebroiler” device, saying broken ceramic tiles inside the units reduced cooking temperatures.

The state said Burger King’s “minimum outcome” must be “fully cooked foods, trained staffs to assure this, and equipment that is reliable and easy to operate.”

In response to Washington public health authorities’ concerns, Burger King said it would immediately inspect its West Coast franchises to see if the company’s protocols were being followed and promised to take corrective measures. The company says its procedures call for cooking hamburger patties to a target temperature of 165 degrees and verifying cooking temperatures at least four times a day.

In 1993, when hamburgers served at Jack in the Box restaurants killed four children and sickened nearly 700 others with E. coli O157:H7 infections, Jack in the Box had been cooking its hamburgers to an internal temperature of 140 degrees, then the federal standard. One year earlier, in 1992, Washington state had raised the minimum temperature for ground beef to 155 degrees, a regulation specifically intended to deal with E. coli.