Japanese health officials have launched a formal investigation into an E. coli outbreak that killed four and sickened more than 70 people after they ate Korean-style steak tartar at a barbecue restaurant chain, reported the state-run Xinhua news agency.

The food poisoning occurred at Yakiniku-zakaya Ebisu restaurant chains in Toyama and Fukui prefectures. Authorities have detected Enterohemorrhagic E. coli bacteria including the O111 and O157 strain.

The restaurant chain operator, Foods Forus Co., admitted the company had not properly monitored the raw meat for contamination for the past two years. “We were not strict enough (about food safety),” said the company president, adding the meat distributor said the beef was safe to eat raw or cooked.