YAKIMA – State health officials investigating a cluster of fatal birth defects in Central Washington have not been able to identify a cause for most of the cases.

The Yakima Herald-Republic reported that the state Department of Health concluded that there was no sole factor to blame for the elevated rates of the anencephaly, a rare neural tube defect.

However, the report released last week said women whose pregnancies were affected by neural tube defects showed low folic acid use compared with other women in Washington.

Between 2010 and 2016, 45 babies in Yakima, Benton and Franklin counties were born with anencephaly. The rate in the three-county area was about four times the national average.

Investigators looked at exposure to nitrates in drinking water, radiation and pesticides. The report said none of those analyses identified a potential cause.

Health officials suspended its investigation in late 2016 into the cause of the elevated rate of anencephaly. They’re now focusing on surveillance, outreach and prevention.