Agencies probing the infestation of non-native, invasive mussels in Lake Mead National Recreation Area said Tuesday that the problem appears to be limited to the lake's Boulder Basin area.

A statement from the National Park Service said divers have found no evidence of adult, invasive mussels at Temple Bar, on the Arizona side of the lake, or in Lake Mohave at Cottonwood Cove and Willow Beach marinas or at the Willow Beach National Fish Hatchery on the Arizona side of Lake Mohave, about 10 miles south of Hoover Dam.

Last week, biologists confirmed that quagga mussels, which are in the same genus as zebra mussels, were found Jan. 6 at the Las Vegas Boat Harbor marina and Lake Mead Marina. Later, invasive mussels turned up in the Nevada Department of Wildlife fish hatchery on Lake Mead.

Like the more commonly known zebra mussels that have been a nuisance in the Great Lakes and Midwest regions since the 1990s, quagga mussels can colonize and obstruct pipes in raw-water delivery systems, costing millions of dollars annually to treat.

Until the quagga mussels were found in Lake Mead this month, they had not been found in the United States west of the Great Lakes or the Mississippi River, according to the U.S. Geological Survey.

A Park Service spokeswoman has said it's possible that both quagga and zebra mussels could have been transported to Lake Mead by a boat or piece of equipment from an infested location. Quagga mussels so far have been the only species found in Lake Mead.

"Additional evaluations are being conducted this week at Callville Bay, Echo Bay, Overton Beach and Hoover Dam," the Park Service statement says.

In the statement, Lake Mead National Recreation Area Superintendent Bill Dickinson said, "Our immediate concern is to ensure we are doing everything we can to stop the spread of invasive mussels. ... Many boaters enjoy both lakes on a regular basis and we need to educate our visitors on the important role they have in stopping the spread to other bodies of water."