ByHeart, a maker of baby formula whose products have been linked to a botulism outbreak that has sickened dozens of infants around the U.S, said that all its formula may have been contaminated.

In a Nov. 24 update posted on the company’s website, ByHeart said that five of 36 samples tested positive for Clostridium botulinum Type A, the bacteria that can cause the potentially deadly illness. 

“Based on these results, we cannot rule out the risk that all ByHeart formula across all product lots may have been contaminated,” the company said in a statement. 

As of Nov. 19, at least 31 cases of reported infant botulism in 15 states have been tied to various shipments of the ByHeart Whole Nutrition infant formula, according to the FDA. No deaths have been reported.

Symptoms of infant botulism, which can take weeks to develop, can include poor feeding, loss of head control, drooping eyelids and a flat facial expression, according to the Centers for Disease Control and Prevention. Babies may also have problems swallowing or breathing.

Clostridium botulinum can be unevenly spread throughout powdered formula, so not all babies who consume a tainted product will fall ill, according to medical experts.

Despite a nationwide recall earlier this month, the FDA said on Nov. 20 that it had received reports that ByHeart formula was still available at major retailers. 

In its Nov. 24 update, ByHeart said that it will provide full refunds for all products purchased through its website on or after Aug. 1. 

“We continue to be focused on finding the root cause, through a rigorous audit of every step of our product development chain, from suppliers and raw ingredients, through to packaging and transportation. This includes testing more product samples,” ByHeart said in a statement posted on its site. 

Some parents of babies sickened from the contaminated formula are suing ByHeart, with two lawsuits filed earlier this month alleging that the product was defective and that the company was negligent in selling it. The parents are seeking financial compensation for medical bills, emotional distress and other harm.

—With reporting by CBS News’ Kiki Intarasuwan