The president of the company whose plant was shut down by the Texas Department of State Health Services has questioned the validity of tests linking celery processed there to a deadly Listeria outbreak.

Kenneth Sanquist Jr. of Sangar Produce & Processing in San Antonio said in a statement Thursday that the state used flawed collection methods, including improper attire, unrefrigerated transport of samples and a delay between collections and lab tests.

Texas health authorities closed a Sangar  processing plant Wednesday after they said tests linked contaminated celery to an outbreak that sickened at least six people, four of them fatally. The state ordered the recall of all of the produce shipped from the plant since January.

The Department of State Health Services says it traced six of 10 confirmed cases of listeriosis to celery processed at the Sangar plant and continues to investigate the source of the other four cases, which include one other death.

Health inspectors said they found unsanitary conditions at the plant, including a condensation drip over a food production area.

A health department spokeswoman said Thursday that the state  stands by its lab results.   She said the department asked the company to close voluntarily but it refused, so the state shut it down and ordered the recall.  She said the state would not have acted had it not been certain.