The battle continues to keep a west-metro lake free of zebra mussels.

After costly and groundbreaking efforts to kill off zebra mussels on Christmas Lake were successful earlier this year, divers searching the lake this week found 10 zebra mussels outside the treated area, the state Department of Natural Resources (DNR) said Friday.

The zebra mussels that were removed had been attached to native mussels, probably since last summer or fall. Crews didn’t know they were there until this week’s more extensive dives, done by the Minnehaha Creek Watershed District, the DNR, Blue Water Science, Waterfront Restorations and the University of Minnesota.

No mussels have been found in the nearly 1-acre area that was treated — the first time zebra mussels have been eliminated from an area of a Minnesota waterway. However, crews will keep monitoring the area and lake, which is near Lake Minnetonka.

“We’re disappointed by the discovery of these additional mussels,” Eric Fieldseth of the Watershed District said in a prepared statement. “However, we’re encouraged to see that no mussels were found in the treatment zone or beyond the immediate vicinity of the treatment area, and we’re exploring our options for addressing what appears to still be a localized infestation.”

The lake was the first in the nation to use Zequanox, a biological pesticide. Then crews used a copper treatment, followed by potash — the third time it’s been used for zebra mussel control in the nation.