ST. PAUL, Minn. (AP) — It turns out that an effort to rid Christmas Lake of zebra mussels has not succeeded.

The Department of Natural Resources said last month that intensive treatments were successful in ridding the public access area to the popular west metropolitan lake of the invasive pests. But divers doing follow-up searches this week found 10 zebra mussels just outside of the treated area.

DNR aquatic invasive species specialist Keegan Lund says those zebra mussels probably had been attached to native mussels in the lake since last summer or fall.

The DNR says it's studying whether any further response is feasible and necessary.

Eric Fieldseth of the Minnehaha Creek Watershed District says he's disappointed. But he says officials are exploring their options for addressing what appears to still be a localized infestation.