Health officials have shut down a San Antonio, Texas food processing plant because it was found to be contaminated by bacteria which may have killed four people.

On Wednesday, the Texas Department of State Health Services ordered SanGar Produce and Processing to stop processing food and recall all products shipped from its plant since January, after lab results of chopped celery at the plant showed Listeria monocytogenes, a bacteria that can cause severe illness.

SanGar questioned the lab's findings and denied any wrongdoing, saying it has had "an excellent record of safety and health."

"The independent testing shows our produce to be absolutely safe, and we are aggressively fighting the state's erroneous findings," said Kenneth Sanquist, president of SanGar.

SanGar processes a wide variety of products including lettuce, peppers, carrots, cucumbers and cut-up fruit, salad, and soup mixes. They are sold in sealed packages to restaurants, hospitals, schools and other large institutions that serve food.

The Texas health department is taking the lead in the investigation, aided by the U.S. Food and Drug Administration and the CDC. The state justified shutting down the SanGar plant, saying the conditions at the plant pose "an immediate and serious threat to human life or health."

The bacteria found in the lab results has been tied to listeriosis, which sickens about 2,500 Americans and contributes to the death of 500 people annually, according to the CDC.