An outbreak of E. coli bacteria has sickened at least 15 people, two seriously, in central New Jersey during the past two weeks, officials said Sunday.

Authorities had yet to determine how and where the victims, mostly children, became infected. Investigators focused on a Taco Bell restaurant in South Plainfield where 11 of the victims ate.

The restaurant, which has been closed voluntarily since Thursday, passed a health code inspection last week and tests were being performed on stool samples from 21 of its employees.

Authorities were still looking for a few other employees from the restaurant and hoped to have results late Sunday or Monday.

"We have taken every precaution, including temporarily closing the restaurant until the investigation is completed, as nothing is more important to us than the health and safety of our customers and employees," said Rob Poetsch, a spokesman for Irvine, Calif.-based Taco Bell Corp.

Seven of the victims were in area hospitals Sunday night. Two had developed a serious condition called hemolytic uremic syndrome that can permanently damage the kidneys. Officials were also investigating four other cases of suspected E. coli infection.

"It's a significant outbreak and it's a serious disease," Middlesex County Director of Health David Papi said.

Two of the confirmed victims were adults, while the others mostly range in age from 7 to 14, Papi said. All the victims were from Middlesex, Monmouth and Somerset counties.

E. coli, short for Escherichia coli, is a common and ordinarily harmless intestinal bacteria. According to the Centers for Disease Control and Prevention, the strain of E. coli that caused the New Jersey infections is often found in the intestines of healthy goats, sheep and cattle, and most infections are associated with undercooked meat.

It can be passed from person to person if people don't take steps such as thoroughly washing their hands.

E. coli may also be found in sprouts or green leafy vegetables such as spinach. Earlier this year, three people died and more than 200 were sickened by a strain of E. coli that was traced to packaged spinach grown in California.