Century-old ice cream company Blue Bell is still struggling to find out how its products got contaminated with listeria, adding more pressure to the massive recall. 

On Tuesday, shelves at grocery stores, which had been occupied by Blue Bell products, now appear empty. 

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"As each day passes, we are getting closer and closer to figuring out how this listeria was introduced into our facilities. ... It's a matter of doing the work and not making excuses," said Blue Bell spokesman Joe Robertson.

The company had recalled all of its products following the outbreak. Three people had already been killed and seven others from Arizona, Texas, Oklahoma and Kansas are still being treated. 

Lois Porter is still grieving over her husband's death last year. Richard Porter complained of intestinal problems followed by fast deteriorating condition.

"His blood was, the doctor said, teeming with listeria," said Porter. "So he had it really bad by the time it was diagnosed."

Richard Porter was among five patients at a Kansas hospital diagnosed to have contracted listeria. All three of the patients who died, were found to have been served Blue Bell ice cream.

Blue Bell CEO Paul Kruse issued a video statement overnight. 

"We are heartbroken about this situation and apologize to all of our loyal Blue Bell fans and customers.  Our entire history has been about making the very best and highest quality ice cream, and we intend to fix this problem," Kruse said.

The company said two of its factories - one in Texas and the other in Oklahoma may have been contaminated.  Blue Bell has hired a team of microbiologists working closely with federal officials to identify the cause of the bacteria.  The company is also stepping up its sanitation system.

Dr. Robert Tauxe of the Centers for Disease Control said the usual cause of listeria is dirty equipment. The bacteria usually comes from soil and water, and may be transmitted and brought in the plant through employees shoes, or spread by employees not properly washing their hands.

The bacteria can survive in both room and cold temperatures. It can stay forever if the area is not cleaned up. 

Blue Bell's recall covers about eight million gallons of its products which include ice cream, yoghurt, sherbet and frozen snacks.  The company says the recall may take about two to three weeks, before its products may be seen back in the shelves. 

TagsBlue Bell Creameries, ice cream, listeria